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bpf: Fix trampoline generation for fmod_ret programs

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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: ae24082331 ("bpf: Introduce BPF_MODIFY_RETURN")
Reported-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: KP Singh <kpsingh@google.com>
Link: https://lore.kernel.org/bpf/20200311003906.3643037-1-ast@kernel.org
This commit is contained in:
Alexei Starovoitov 2020-03-10 17:39:06 -07:00 committed by Daniel Borkmann
parent babf316409
commit 13fac1d851
1 changed files with 5 additions and 26 deletions
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31
arch/x86/net/bpf_jit_comp.c
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@ -1449,23 +1449,6 @@ static int emit_cond_near_jump(u8 **pprog, void *func, void *ip, u8 jmp_cond)
return 0;
}
static int emit_mod_ret_check_imm8(u8 **pprog, int value)
{
u8 *prog = *pprog;
int cnt = 0;
if (!is_imm8(value))
return -EINVAL;
if (value == 0)
EMIT2(0x85, add_2reg(0xC0, BPF_REG_0, BPF_REG_0));
else
EMIT3(0x83, add_1reg(0xF8, BPF_REG_0), value);
*pprog = prog;
return 0;
}
static int invoke_bpf(const struct btf_func_model *m, u8 **pprog,
struct bpf_tramp_progs *tp, int stack_size)
{
@ -1485,7 +1468,7 @@ static int invoke_bpf_mod_ret(const struct btf_func_model *m, u8 **pprog,
u8 **branches)
{
u8 *prog = *pprog;
int i;
int i, cnt = 0;
/* The first fmod_ret program will receive a garbage return value.
* Set this to 0 to avoid confusing the program.
@ -1496,16 +1479,12 @@ static int invoke_bpf_mod_ret(const struct btf_func_model *m, u8 **pprog,
if (invoke_bpf_prog(m, &prog, tp->progs[i], stack_size, true))
return -EINVAL;
/* Generate a branch:
*
* if (ret != 0)
/* mod_ret prog stored return value into [rbp - 8]. Emit:
* if (*(u64 *)(rbp - 8) != 0)
* goto do_fexit;
*
* If needed this can be extended to any integer value which can
* be passed by user-space when the program is loaded.
*/
if (emit_mod_ret_check_imm8(&prog, 0))
return -EINVAL;
/* cmp QWORD PTR [rbp - 0x8], 0x0 */
EMIT4(0x48, 0x83, 0x7d, 0xf8); EMIT1(0x00);
/* Save the location of the branch and Generate 6 nops
* (4 bytes for an offset and 2 bytes for the jump) These nops