On 3/11/20 1:39 AM, Alexei Starovoitov wrote:
fmod_ret progs are emitted as: start = __bpf_prog_enter(); call fmod_ret *(u64 *)(rbp - 8) = rax __bpf_prog_exit(, start); test eax, eax jne do_fexit That 'test eax, eax' is working by accident. The compiler is free to use rax inside __bpf_prog_exit() or inside functions that __bpf_prog_exit() is calling. Which caused "test_progs -t modify_return" to sporadically fail depending on compiler version and kconfig. Fix it by using 'cmp [rbp - 8], 0' instead of 'test eax, eax'. Fixes: ae24082331d9 ("bpf: Introduce BPF_MODIFY_RETURN") Reported-by: Andrii Nakryiko <andriin@xxxxxx> Acked-by: Andrii Nakryiko <andriin@xxxxxx> Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Applied, thanks!