Writing an eBPF bound check using embedded assembly

Question

I have a bound check statement that is optimized by clang and as a result verifier doesn't accept the program. I would like to write it using embedded assembly. I am not sure how to do it. I know that the __asm__ block given bellow will compile to if r3 > r1 goto pc+5 . What I don't know is how to reference labels in the embedded assembly.

Writing eBPF assembly is very different than x86 (actually feels more intuitive). I wonder if there is a manual or a document or source file in Clang that can help me understand the syntax.

__asm__("if %0 > %1 goto 5  \n\t"
:
: "r"(tmp_ptr), "r"(data_end)
:
);

pchaigno · Accepted Answer · 2025-09-23 09:59:17Z

You can find an example of how to do that in the kernel BPF selftests:

$ git grep -hC5 '\bif\b.*goto %' tools/testing/selftests/bpf/

#define __bpf_cmp(LHS, OP, PRED, RHS, DEFAULT)                              \
    ({                                                                      \
        __label__ l_true;                                                   \
        bool ret = DEFAULT;                                                 \
        asm volatile goto("if %[lhs] " OP " %[rhs] goto %l[l_true]"         \
                  :: [lhs] "r"((short)LHS), [rhs] PRED (RHS) :: l_true);    \
        ret = !DEFAULT;                                                     \
l_true:                                                                     \
        ret;                                                                \
    })

So you can just refer to the C label from the inline asm, with %l[label].

Collectives™ on Stack Overflow

Writing an eBPF bound check using embedded assembly

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related