eBPF verifier¶

The safety of the eBPF program is determined in two steps.

First step does DAG check to disallow loops and other CFG validation. In particular it will detect programs that have unreachable instructions. (though classic BPF checker allows them)

Second step starts from the first insn and descends all possible paths. It simulates execution of every insn and observes the state change of registers and stack.

At the start of the program the register R1 contains a pointer to context and has type PTR_TO_CTX. If verifier sees an insn that does R2=R1, then R2 has now type PTR_TO_CTX as well and can be used on the right hand side of expression. If R1=PTR_TO_CTX and insn is R2=R1+R1, then R2=SCALAR_VALUE, since addition of two valid pointers makes invalid pointer. (In ‘secure’ mode verifier will reject any type of pointer arithmetic to make sure that kernel addresses don’t leak to unprivileged users)

If register was never written to, it’s not readable:

bpf_mov R0 = R2
bpf_exit

will be rejected, since R2 is unreadable at the start of the program.

After kernel function call, R1-R5 are reset to unreadable and R0 has a return type of the function.

Since R6-R9 are callee saved, their state is preserved across the call.

bpf_mov R6 = 1
bpf_call foo
bpf_mov R0 = R6
bpf_exit

is a correct program. If there was R1 instead of R6, it would have been rejected.

load/store instructions are allowed only with registers of valid types, which are PTR_TO_CTX, PTR_TO_MAP, PTR_TO_STACK. They are bounds and alignment checked. For example:

bpf_mov R1 = 1
bpf_mov R2 = 2
bpf_xadd *(u32 *)(R1 + 3) += R2
bpf_exit

will be rejected, since R1 doesn’t have a valid pointer type at the time of execution of instruction bpf_xadd.

At the start R1 type is PTR_TO_CTX (a pointer to generic struct bpf_context) A callback is used to customize verifier to restrict eBPF program access to only certain fields within ctx structure with specified size and alignment.

For example, the following insn:

bpf_ld R0 = *(u32 *)(R6 + 8)

intends to load a word from address R6 + 8 and store it into R0 If R6=PTR_TO_CTX, via is_valid_access() callback the verifier will know that offset 8 of size 4 bytes can be accessed for reading, otherwise the verifier will reject the program. If R6=PTR_TO_STACK, then access should be aligned and be within stack bounds, which are [-MAX_BPF_STACK, 0). In this example offset is 8, so it will fail verification, since it’s out of bounds.

The verifier will allow eBPF program to read data from stack only after it wrote into it.

Classic BPF verifier does similar check with M[0-15] memory slots. For example:

bpf_ld R0 = *(u32 *)(R10 - 4)
bpf_exit

is invalid program. Though R10 is correct read-only register and has type PTR_TO_STACK and R10 - 4 is within stack bounds, there were no stores into that location.

Pointer register spill/fill is tracked as well, since four (R6-R9) callee saved registers may not be enough for some programs.

Allowed function calls are customized with bpf_verifier_ops->get_func_proto() The eBPF verifier will check that registers match argument constraints. After the call register R0 will be set to return type of the function.

Function calls is a main mechanism to extend functionality of eBPF programs. Socket filters may let programs to call one set of functions, whereas tracing filters may allow completely different set.

If a function made accessible to eBPF program, it needs to be thought through from safety point of view. The verifier will guarantee that the function is called with valid arguments.

seccomp vs socket filters have different security restrictions for classic BPF. Seccomp solves this by two stage verifier: classic BPF verifier is followed by seccomp verifier. In case of eBPF one configurable verifier is shared for all use cases.

See details of eBPF verifier in kernel/bpf/verifier.c

Register value tracking¶

In order to determine the safety of an eBPF program, the verifier must track the range of possible values in each register and also in each stack slot. This is done with struct bpf_reg_state, defined in include/linux/ bpf_verifier.h, which unifies tracking of scalar and pointer values. Each register state has a type, which is either NOT_INIT (the register has not been written to), SCALAR_VALUE (some value which is not usable as a pointer), or a pointer type. The types of pointers describe their base, as follows:

PTR_TO_CTX
Pointer to bpf_context.

CONST_PTR_TO_MAP
Pointer to struct bpf_map. “Const” because arithmetic on these pointers is forbidden.

PTR_TO_MAP_VALUE
Pointer to the value stored in a map element.

PTR_TO_MAP_VALUE_OR_NULL
Either a pointer to a map value, or NULL; map accesses (see BPF maps) return this type, which becomes a PTR_TO_MAP_VALUE when checked != NULL. Arithmetic on these pointers is forbidden.

PTR_TO_STACK
Frame pointer.

PTR_TO_PACKET
skb->data.

PTR_TO_PACKET_END
skb->data + headlen; arithmetic forbidden.

PTR_TO_SOCKET
Pointer to struct bpf_sock_ops, implicitly refcounted.

PTR_TO_SOCKET_OR_NULL
Either a pointer to a socket, or NULL; socket lookup returns this type, which becomes a PTR_TO_SOCKET when checked != NULL. PTR_TO_SOCKET is reference-counted, so programs must release the reference through the socket release function before the end of the program. Arithmetic on these pointers is forbidden.

However, a pointer may be offset from this base (as a result of pointer arithmetic), and this is tracked in two parts: the ‘fixed offset’ and ‘variable offset’. The former is used when an exactly-known value (e.g. an immediate operand) is added to a pointer, while the latter is used for values which are not exactly known. The variable offset is also used in SCALAR_VALUEs, to track the range of possible values in the register.

The verifier’s knowledge about the variable offset consists of:

minimum and maximum values as unsigned
minimum and maximum values as signed
knowledge of the values of individual bits, in the form of a ‘tnum’: a u64 ‘mask’ and a u64 ‘value’. 1s in the mask represent bits whose value is unknown; 1s in the value represent bits known to be 1. Bits known to be 0 have 0 in both mask and value; no bit should ever be 1 in both. For example, if a byte is read into a register from memory, the register’s top 56 bits are known zero, while the low 8 are unknown - which is represented as the tnum (0x0; 0xff). If we then OR this with 0x40, we get (0x40; 0xbf), then if we add 1 we get (0x0; 0x1ff), because of potential carries.

Besides arithmetic, the register state can also be updated by conditional branches. For instance, if a SCALAR_VALUE is compared > 8, in the ‘true’ branch it will have a umin_value (unsigned minimum value) of 9, whereas in the ‘false’ branch it will have a umax_value of 8. A signed compare (with BPF_JSGT or BPF_JSGE) would instead update the signed minimum/maximum values. Information from the signed and unsigned bounds can be combined; for instance if a value is first tested < 8 and then tested s> 4, the verifier will conclude that the value is also > 4 and s< 8, since the bounds prevent crossing the sign boundary.

PTR_TO_PACKETs with a variable offset part have an ‘id’, which is common to all pointers sharing that same variable offset. This is important for packet range checks: after adding a variable to a packet pointer register A, if you then copy it to another register B and then add a constant 4 to A, both registers will share the same ‘id’ but the A will have a fixed offset of +4. Then if A is bounds-checked and found to be less than a PTR_TO_PACKET_END, the register B is now known to have a safe range of at least 4 bytes. See ‘Direct packet access’, below, for more on PTR_TO_PACKET ranges.

The ‘id’ field is also used on PTR_TO_MAP_VALUE_OR_NULL, common to all copies of the pointer returned from a map lookup. This means that when one copy is checked and found to be non-NULL, all copies can become PTR_TO_MAP_VALUEs. As well as range-checking, the tracked information is also used for enforcing alignment of pointer accesses. For instance, on most systems the packet pointer is 2 bytes after a 4-byte alignment. If a program adds 14 bytes to that to jump over the Ethernet header, then reads IHL and adds (IHL * 4), the resulting pointer will have a variable offset known to be 4n+2 for some n, so adding the 2 bytes (NET_IP_ALIGN) gives a 4-byte alignment and so word-sized accesses through that pointer are safe. The ‘id’ field is also used on PTR_TO_SOCKET and PTR_TO_SOCKET_OR_NULL, common to all copies of the pointer returned from a socket lookup. This has similar behaviour to the handling for PTR_TO_MAP_VALUE_OR_NULL->PTR_TO_MAP_VALUE, but it also handles reference tracking for the pointer. PTR_TO_SOCKET implicitly represents a reference to the corresponding struct sock. To ensure that the reference is not leaked, it is imperative to NULL-check the reference and in the non-NULL case, and pass the valid reference to the socket release function.

Direct packet access¶

In cls_bpf and act_bpf programs the verifier allows direct access to the packet data via skb->data and skb->data_end pointers. Ex:

 r4 = *(u32 *)(r1 +80)  /* load skb->data_end */
 r3 = *(u32 *)(r1 +76)  /* load skb->data */
 r5 = r3
 r5 += 14
 if r5 > r4 goto pc+16
R1=ctx R3=pkt(id=0,off=0,r=14) R4=pkt_end R5=pkt(id=0,off=14,r=14) R10=fp
 r0 = *(u16 *)(r3 +12) /* access 12 and 13 bytes of the packet */

this 2byte load from the packet is safe to do, since the program author did check if (skb->data + 14 > skb->data_end) goto err at insn #5 which means that in the fall-through case the register R3 (which points to skb->data) has at least 14 directly accessible bytes. The verifier marks it as R3=pkt(id=0,off=0,r=14). id=0 means that no additional variables were added to the register. off=0 means that no additional constants were added. r=14 is the range of safe access which means that bytes [R3, R3 + 14) are ok. Note that R5 is marked as R5=pkt(id=0,off=14,r=14). It also points to the packet data, but constant 14 was added to the register, so it now points to skb->data + 14 and accessible range is [R5, R5 + 14 - 14) which is zero bytes.

More complex packet access may look like:

R0=inv1 R1=ctx R3=pkt(id=0,off=0,r=14) R4=pkt_end R5=pkt(id=0,off=14,r=14) R10=fp
 r0 = *(u8 *)(r3 +7) /* load 7th byte from the packet */
 r4 = *(u8 *)(r3 +12)
 r4 *= 14
 r3 = *(u32 *)(r1 +76) /* load skb->data */
 r3 += r4
 r2 = r1
 r2 <<= 48
 r2 >>= 48
 r3 += r2
 r2 = r3
 r2 += 8
 r1 = *(u32 *)(r1 +80) /* load skb->data_end */
 if r2 > r1 goto pc+2
R0=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R1=pkt_end R2=pkt(id=2,off=8,r=8) R3=pkt(id=2,off=0,r=8) R4=inv(id=0,umax_value=3570,var_off=(0x0; 0xfffe)) R5=pkt(id=0,off=14,r=14) R10=fp
 r1 = *(u8 *)(r3 +4)

The state of the register R3 is R3=pkt(id=2,off=0,r=8) id=2 means that two r3 += rX instructions were seen, so r3 points to some offset within a packet and since the program author did if (r3 + 8 > r1) goto err at insn #18, the safe range is [R3, R3 + 8). The verifier only allows ‘add’/’sub’ operations on packet registers. Any other operation will set the register state to ‘SCALAR_VALUE’ and it won’t be available for direct packet access.

Operation r3 += rX may overflow and become less than original skb->data, therefore the verifier has to prevent that. So when it sees r3 += rX instruction and rX is more than 16-bit value, any subsequent bounds-check of r3 against skb->data_end will not give us ‘range’ information, so attempts to read through the pointer will give “invalid access to packet” error.

Ex. after insn r4 = *(u8 *)(r3 +12) (insn #7 above) the state of r4 is R4=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) which means that upper 56 bits of the register are guaranteed to be zero, and nothing is known about the lower 8 bits. After insn r4 *= 14 the state becomes R4=inv(id=0,umax_value=3570,var_off=(0x0; 0xfffe)), since multiplying an 8-bit value by constant 14 will keep upper 52 bits as zero, also the least significant bit will be zero as 14 is even. Similarly r2 >>= 48 will make R2=inv(id=0,umax_value=65535,var_off=(0x0; 0xffff)), since the shift is not sign extending. This logic is implemented in adjust_reg_min_max_vals() function, which calls adjust_ptr_min_max_vals() for adding pointer to scalar (or vice versa) and adjust_scalar_min_max_vals() for operations on two scalars.

The end result is that bpf program author can access packet directly using normal C code as:

void *data = (void *)(long)skb->data;
void *data_end = (void *)(long)skb->data_end;
struct eth_hdr *eth = data;
struct iphdr *iph = data + sizeof(*eth);
struct udphdr *udp = data + sizeof(*eth) + sizeof(*iph);

if (data + sizeof(*eth) + sizeof(*iph) + sizeof(*udp) > data_end)
        return 0;
if (eth->h_proto != htons(ETH_P_IP))
        return 0;
if (iph->protocol != IPPROTO_UDP || iph->ihl != 5)
        return 0;
if (udp->dest == 53 || udp->source == 9)
        ...;

which makes such programs easier to write comparing to LD_ABS insn and significantly faster.

Pruning¶

The verifier does not actually walk all possible paths through the program. For each new branch to analyse, the verifier looks at all the states it’s previously been in when at this instruction. If any of them contain the current state as a subset, the branch is ‘pruned’ - that is, the fact that the previous state was accepted implies the current state would be as well. For instance, if in the previous state, r1 held a packet-pointer, and in the current state, r1 holds a packet-pointer with a range as long or longer and at least as strict an alignment, then r1 is safe. Similarly, if r2 was NOT_INIT before then it can’t have been used by any path from that point, so any value in r2 (including another NOT_INIT) is safe. The implementation is in the function regsafe(). Pruning considers not only the registers but also the stack (and any spilled registers it may hold). They must all be safe for the branch to be pruned. This is implemented in states_equal().

Some technical details about state pruning implementation could be found below.

Register liveness tracking¶

In order to make state pruning effective, liveness state is tracked for each register and stack slot. The basic idea is to track which registers and stack slots are actually used during subseqeuent execution of the program, until program exit is reached. Registers and stack slots that were never used could be removed from the cached state thus making more states equivalent to a cached state. This could be illustrated by the following program:

call bpf_get_prandom_u32()
r1 = 0
if r0 == 0 goto +1
r0 = 1
--- checkpoint ---
r0 = r1
exit

Suppose that a state cache entry is created at instruction #4 (such entries are also called “checkpoints” in the text below). The verifier could reach the instruction with one of two possible register states:

r0 = 1, r1 = 0
r0 = 0, r1 = 0

However, only the value of register r1 is important to successfully finish verification. The goal of the liveness tracking algorithm is to spot this fact and figure out that both states are actually equivalent.

Data structures¶

Liveness is tracked using the following data structures:

enum bpf_reg_liveness {
      REG_LIVE_NONE = 0,
      REG_LIVE_READ32 = 0x1,
      REG_LIVE_READ64 = 0x2,
      REG_LIVE_READ = REG_LIVE_READ32 | REG_LIVE_READ64,
      REG_LIVE_WRITTEN = 0x4,
      REG_LIVE_DONE = 0x8,
};

struct bpf_reg_state {
      ...
      struct bpf_reg_state *parent;
      ...
      enum bpf_reg_liveness live;
      ...
};

struct bpf_stack_state {
      struct bpf_reg_state spilled_ptr;
      ...
};

struct bpf_func_state {
      struct bpf_reg_state regs[MAX_BPF_REG];
      ...
      struct bpf_stack_state *stack;
}

struct bpf_verifier_state {
      struct bpf_func_state *frame[MAX_CALL_FRAMES];
      struct bpf_verifier_state *parent;
      ...
}

REG_LIVE_NONE is an initial value assigned to ->live fields upon new verifier state creation;
REG_LIVE_WRITTEN means that the value of the register (or stack slot) is defined by some instruction verified between this verifier state’s parent and verifier state itself;
REG_LIVE_READ{32,64} means that the value of the register (or stack slot) is read by a some child state of this verifier state;
REG_LIVE_DONE is a marker used by clean_verifier_state() to avoid processing same verifier state multiple times and for some sanity checks;
->live field values are formed by combining enum bpf_reg_liveness values using bitwise or.

Register parentage chains¶

In order to propagate information between parent and child states, a register parentage chain is established. Each register or stack slot is linked to a corresponding register or stack slot in its parent state via a ->parent pointer. This link is established upon state creation in is_state_visited() and might be modified by set_callee_state() called from __check_func_call().

The rules for correspondence between registers / stack slots are as follows:

For the current stack frame, registers and stack slots of the new state are linked to the registers and stack slots of the parent state with the same indices.
For the outer stack frames, only callee saved registers (r6-r9) and stack slots are linked to the registers and stack slots of the parent state with the same indices.
When function call is processed a new struct bpf_func_state instance is allocated, it encapsulates a new set of registers and stack slots. For this new frame, parent links for r6-r9 and stack slots are set to nil, parent links for r1-r5 are set to match caller r1-r5 parent links.

This could be illustrated by the following diagram (arrows stand for ->parent pointers):

    ...                    ; Frame #0, some instructions
--- checkpoint #0 ---
1 : r6 = 42                ; Frame #0
--- checkpoint #1 ---
2 : call foo()             ; Frame #0
    ...                    ; Frame #1, instructions from foo()
--- checkpoint #2 ---
    ...                    ; Frame #1, instructions from foo()
--- checkpoint #3 ---
    exit                   ; Frame #1, return from foo()
3 : r1 = r6                ; Frame #0  <- current state

           +-------------------------------+-------------------------------+
           |           Frame #0            |           Frame #1            |
Checkpoint +-------------------------------+-------------------------------+
#0         | r0 | r1-r5 | r6-r9 | fp-8 ... |
           +-------------------------------+
              ^    ^       ^       ^
              |    |       |       |
Checkpoint +-------------------------------+
#1         | r0 | r1-r5 | r6-r9 | fp-8 ... |
           +-------------------------------+
                   ^       ^       ^
                   |_______|_______|_______________
                           |       |               |
             nil  nil      |       |               |      nil     nil
              |    |       |       |               |       |       |
Checkpoint +-------------------------------+-------------------------------+
#2         | r0 | r1-r5 | r6-r9 | fp-8 ... | r0 | r1-r5 | r6-r9 | fp-8 ... |
           +-------------------------------+-------------------------------+
                           ^       ^               ^       ^       ^
             nil  nil      |       |               |       |       |
              |    |       |       |               |       |       |
Checkpoint +-------------------------------+-------------------------------+
#3         | r0 | r1-r5 | r6-r9 | fp-8 ... | r0 | r1-r5 | r6-r9 | fp-8 ... |
           +-------------------------------+-------------------------------+
                           ^       ^
             nil  nil      |       |
              |    |       |       |
Current    +-------------------------------+
state      | r0 | r1-r5 | r6-r9 | fp-8 ... |
           +-------------------------------+
                           \
                             r6 read mark is propagated via these links
                             all the way up to checkpoint #1.
                             The checkpoint #1 contains a write mark for r6
                             because of instruction (1), thus read propagation
                             does not reach checkpoint #0 (see section below).

Liveness marks tracking¶

For each processed instruction, the verifier tracks read and written registers and stack slots. The main idea of the algorithm is that read marks propagate back along the state parentage chain until they hit a write mark, which ‘screens off’ earlier states from the read. The information about reads is propagated by function mark_reg_read() which could be summarized as follows:

mark_reg_read(struct bpf_reg_state *state, ...):
    parent = state->parent
    while parent:
        if state->live & REG_LIVE_WRITTEN:
            break
        if parent->live & REG_LIVE_READ64:
            break
        parent->live |= REG_LIVE_READ64
        state = parent
        parent = state->parent

Notes:

The read marks are applied to the parent state while write marks are applied to the current state. The write mark on a register or stack slot means that it is updated by some instruction in the straight-line code leading from the parent state to the current state.
Details about REG_LIVE_READ32 are omitted.
Function propagate_liveness() (see section Read marks propagation for cache hits) might override the first parent link. Please refer to the comments in the propagate_liveness() and mark_reg_read() source code for further details.

Because stack writes could have different sizes REG_LIVE_WRITTEN marks are applied conservatively: stack slots are marked as written only if write size corresponds to the size of the register, e.g. see function save_register_state().

Consider the following example:

(*u64)(r10 - 8) = 0   ; define 8 bytes of fp-8
--- checkpoint #0 ---
(*u32)(r10 - 8) = 1   ; redefine lower 4 bytes
r1 = (*u32)(r10 - 8)  ; read lower 4 bytes defined at (1)
r2 = (*u32)(r10 - 4)  ; read upper 4 bytes defined at (0)

As stated above, the write at (1) does not count as REG_LIVE_WRITTEN. Should it be otherwise, the algorithm above wouldn’t be able to propagate the read mark from (3) to checkpoint #0.

Once the BPF_EXIT instruction is reached update_branch_counts() is called to update the ->branches counter for each verifier state in a chain of parent verifier states. When the ->branches counter reaches zero the verifier state becomes a valid entry in a set of cached verifier states.

Each entry of the verifier states cache is post-processed by a function clean_live_states(). This function marks all registers and stack slots without REG_LIVE_READ{32,64} marks as NOT_INIT or STACK_INVALID. Registers/stack slots marked in this way are ignored in function stacksafe() called from states_equal() when a state cache entry is considered for equivalence with a current state.

Now it is possible to explain how the example from the beginning of the section works:

call bpf_get_prandom_u32()
r1 = 0
if r0 == 0 goto +1
r0 = 1
--- checkpoint[0] ---
r0 = r1
exit

At instruction #2 branching point is reached and state { r0 == 0, r1 == 0, pc == 4 } is pushed to states processing queue (pc stands for program counter).
At instruction #4:
- checkpoint[0] states cache entry is created: { r0 == 1, r1 == 0, pc == 4 };
- checkpoint[0].r0 is marked as written;
- checkpoint[0].r1 is marked as read;
At instruction #5 exit is reached and checkpoint[0] can now be processed by clean_live_states(). After this processing checkpoint[0].r1 has a read mark and all other registers and stack slots are marked as NOT_INIT or STACK_INVALID
The state { r0 == 0, r1 == 0, pc == 4 } is popped from the states queue and is compared against a cached state { r1 == 0, pc == 4 }, the states are considered equivalent.

Read marks propagation for cache hits¶

Another point is the handling of read marks when a previously verified state is found in the states cache. Upon cache hit verifier must behave in the same way as if the current state was verified to the program exit. This means that all read marks, present on registers and stack slots of the cached state, must be propagated over the parentage chain of the current state. Example below shows why this is important. Function propagate_liveness() handles this case.

Consider the following state parentage chain (S is a starting state, A-E are derived states, -> arrows show which state is derived from which):

               r1 read
        <-------------                A[r1] == 0
                                      C[r1] == 0
  S ---> A ---> B ---> exit           E[r1] == 1
  |
  ` ---> C ---> D
  |
  ` ---> E      ^
                |___   suppose all these
         ^           states are at insn #Y
         |
  suppose all these
states are at insn #X

Chain of states S -> A -> B -> exit is verified first.
While B -> exit is verified, register r1 is read and this read mark is propagated up to state A.
When chain of states C -> D is verified the state D turns out to be equivalent to state B.
The read mark for r1 has to be propagated to state C, otherwise state C might get mistakenly marked as equivalent to state E even though values for register r1 differ between C and E.

Understanding eBPF verifier messages¶

The following are few examples of invalid eBPF programs and verifier error messages as seen in the log:

Program with unreachable instructions:

static struct bpf_insn prog[] = {
BPF_EXIT_INSN(),
BPF_EXIT_INSN(),
};

Error:

unreachable insn 1

Program that reads uninitialized register:

BPF_MOV64_REG(BPF_REG_0, BPF_REG_2),
BPF_EXIT_INSN(),

Error:

0: (bf) r0 = r2
R2 !read_ok

Program that doesn’t initialize R0 before exiting:

BPF_MOV64_REG(BPF_REG_2, BPF_REG_1),
BPF_EXIT_INSN(),

Error:

0: (bf) r2 = r1
1: (95) exit
R0 !read_ok

Program that accesses stack out of bounds:

BPF_ST_MEM(BPF_DW, BPF_REG_10, 8, 0),
BPF_EXIT_INSN(),

Error:

0: (7a) *(u64 *)(r10 +8) = 0
invalid stack off=8 size=8

Program that doesn’t initialize stack before passing its address into function:

BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
BPF_LD_MAP_FD(BPF_REG_1, 0),
BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
BPF_EXIT_INSN(),

Error:

(bf) r2 = r10
(07) r2 += -8
(b7) r1 = 0x0
(85) call 1
invalid indirect read from stack off -8+0 size 8

Program that uses invalid map_fd=0 while calling to map_lookup_elem() function:

BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
BPF_LD_MAP_FD(BPF_REG_1, 0),
BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
BPF_EXIT_INSN(),

Error:

(7a) *(u64 *)(r10 -8) = 0
(bf) r2 = r10
(07) r2 += -8
(b7) r1 = 0x0
(85) call 1
fd 0 is not pointing to valid bpf_map

Program that doesn’t check return value of map_lookup_elem() before accessing map element:

BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
BPF_LD_MAP_FD(BPF_REG_1, 0),
BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 0),
BPF_EXIT_INSN(),

Error:

(7a) *(u64 *)(r10 -8) = 0
(bf) r2 = r10
(07) r2 += -8
(b7) r1 = 0x0
(85) call 1
(7a) *(u64 *)(r0 +0) = 0
R0 invalid mem access 'map_value_or_null'

Program that correctly checks map_lookup_elem() returned value for NULL, but accesses the memory with incorrect alignment:

BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
BPF_LD_MAP_FD(BPF_REG_1, 0),
BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 1),
BPF_ST_MEM(BPF_DW, BPF_REG_0, 4, 0),
BPF_EXIT_INSN(),

Error:

(7a) *(u64 *)(r10 -8) = 0
(bf) r2 = r10
(07) r2 += -8
(b7) r1 = 1
(85) call 1
(15) if r0 == 0x0 goto pc+1
 R0=map_ptr R10=fp
(7a) *(u64 *)(r0 +4) = 0
misaligned access off 4 size 8

Program that correctly checks map_lookup_elem() returned value for NULL and accesses memory with correct alignment in one side of ‘if’ branch, but fails to do so in the other side of ‘if’ branch:

BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
BPF_LD_MAP_FD(BPF_REG_1, 0),
BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 2),
BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 0),
BPF_EXIT_INSN(),
BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 1),
BPF_EXIT_INSN(),

Error:

(7a) *(u64 *)(r10 -8) = 0
(bf) r2 = r10
(07) r2 += -8
(b7) r1 = 1
(85) call 1
(15) if r0 == 0x0 goto pc+2
 R0=map_ptr R10=fp
(7a) *(u64 *)(r0 +0) = 0
(95) exit

from 5 to 8: R0=imm0 R10=fp
(7a) *(u64 *)(r0 +0) = 1
R0 invalid mem access 'imm'

Program that performs a socket lookup then sets the pointer to NULL without checking it:

BPF_MOV64_IMM(BPF_REG_2, 0),
BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_2, -8),
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
BPF_MOV64_IMM(BPF_REG_3, 4),
BPF_MOV64_IMM(BPF_REG_4, 0),
BPF_MOV64_IMM(BPF_REG_5, 0),
BPF_EMIT_CALL(BPF_FUNC_sk_lookup_tcp),
BPF_MOV64_IMM(BPF_REG_0, 0),
BPF_EXIT_INSN(),

Error:

(b7) r2 = 0
(63) *(u32 *)(r10 -8) = r2
(bf) r2 = r10
(07) r2 += -8
(b7) r3 = 4
(b7) r4 = 0
(b7) r5 = 0
(85) call bpf_sk_lookup_tcp#65
(b7) r0 = 0
(95) exit
Unreleased reference id=1, alloc_insn=7

Program that performs a socket lookup but does not NULL-check the returned value:

BPF_MOV64_IMM(BPF_REG_2, 0),
BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_2, -8),
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
BPF_MOV64_IMM(BPF_REG_3, 4),
BPF_MOV64_IMM(BPF_REG_4, 0),
BPF_MOV64_IMM(BPF_REG_5, 0),
BPF_EMIT_CALL(BPF_FUNC_sk_lookup_tcp),
BPF_EXIT_INSN(),

Error:

(b7) r2 = 0
(63) *(u32 *)(r10 -8) = r2
(bf) r2 = r10
(07) r2 += -8
(b7) r3 = 4
(b7) r4 = 0
(b7) r5 = 0
(85) call bpf_sk_lookup_tcp#65
(95) exit
Unreleased reference id=1, alloc_insn=7

The Linux Kernel

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eBPF verifier¶

Register value tracking¶

Direct packet access¶

Pruning¶

Register liveness tracking¶

Data structures¶

Register parentage chains¶

Liveness marks tracking¶

Read marks propagation for cache hits¶

Understanding eBPF verifier messages¶