Commit df0cb5cb authored by KaFai Wan's avatar KaFai Wan Committed by Alexei Starovoitov
Browse files

bpf: Allow fall back to interpreter for programs with stack size <= 512 



OpenWRT users reported regression on ARMv6 devices after updating to latest
HEAD, where tcpdump filter:

tcpdump "not ether host 3c37121a2b3c and not ether host 184ecbca2a3a \
and not ether host 14130b4d3f47 and not ether host f0f61cf440b7 \
and not ether host a84b4dedf471 and not ether host d022be17e1d7 \
and not ether host 5c497967208b and not ether host 706655784d5b"

fails with warning: "Kernel filter failed: No error information"
when using config:
 # CONFIG_BPF_JIT_ALWAYS_ON is not set
 CONFIG_BPF_JIT_DEFAULT_ON=y

The issue arises because commits:
1. "bpf: Fix array bounds error with may_goto" changed default runtime to
   __bpf_prog_ret0_warn when jit_requested = 1
2. "bpf: Avoid __bpf_prog_ret0_warn when jit fails" returns error when
   jit_requested = 1 but jit fails

This change restores interpreter fallback capability for BPF programs with
stack size <= 512 bytes when jit fails.

Reported-by: default avatarFelix Fietkau <nbd@nbd.name>
Closes: https://lore.kernel.org/bpf/2e267b4b-0540-45d8-9310-e127bf95fc63@nbd.name/


Fixes: 6ebc5030 ("bpf: Fix array bounds error with may_goto")
Signed-off-by: default avatarKaFai Wan <kafai.wan@linux.dev>
Acked-by: default avatarEduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250909144614.2991253-1-kafai.wan@linux.dev


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent 0d80e7f9

kernel/bpf/core.c

+9 −7
Original line number Diff line number Diff line
@@ -2366,8 +2366,7 @@ static unsigned int __bpf_prog_ret0_warn(const void *ctx, 
					 const struct bpf_insn *insn)

{

	/* If this handler ever gets executed, then BPF_JIT_ALWAYS_ON

	 * is not working properly, or interpreter is being used when

	 * prog->jit_requested is not 0, so warn about it!

	 * is not working properly, so warn about it!

	 */

	WARN_ON_ONCE(1);

	return 0;
@@ -2468,8 +2467,9 @@ static int bpf_check_tail_call(const struct bpf_prog *fp) 
	return ret;

}



static void bpf_prog_select_func(struct bpf_prog *fp)

static bool bpf_prog_select_interpreter(struct bpf_prog *fp)

{

	bool select_interpreter = false;

#ifndef CONFIG_BPF_JIT_ALWAYS_ON

	u32 stack_depth = max_t(u32, fp->aux->stack_depth, 1);

	u32 idx = (round_up(stack_depth, 32) / 32) - 1;
@@ -2478,15 +2478,16 @@ static void bpf_prog_select_func(struct bpf_prog *fp) 
	 * But for non-JITed programs, we don't need bpf_func, so no bounds

	 * check needed.

	 */

	if (!fp->jit_requested &&

	    !WARN_ON_ONCE(idx >= ARRAY_SIZE(interpreters))) {

	if (idx < ARRAY_SIZE(interpreters)) {

		fp->bpf_func = interpreters[idx];

		select_interpreter = true;

	} else {

		fp->bpf_func = __bpf_prog_ret0_warn;

	}

#else

	fp->bpf_func = __bpf_prog_ret0_warn;

#endif

	return select_interpreter;

}



/**
@@ -2505,7 +2506,7 @@ struct bpf_prog *bpf_prog_select_runtime(struct bpf_prog *fp, int *err) 
	/* In case of BPF to BPF calls, verifier did all the prep

	 * work with regards to JITing, etc.

	 */

	bool jit_needed = fp->jit_requested;

	bool jit_needed = false;



	if (fp->bpf_func)

		goto finalize;
@@ -2514,7 +2515,8 @@ struct bpf_prog *bpf_prog_select_runtime(struct bpf_prog *fp, int *err) 
	    bpf_prog_has_kfunc_call(fp))

		jit_needed = true;



	bpf_prog_select_func(fp);

	if (!bpf_prog_select_interpreter(fp))

		jit_needed = true;



	/* eBPF JITs can rewrite the program in case constant

	 * blinding is active. However, in case of error during