[PATCH bpf v3 1/1] bpf: Allow fall back to interpreter for programs with stack size <= 512

[KaFai Wan <kafai.wan@linux.dev>]

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: Felix Fietkau <nbd@nbd.name>
Closes: https://lore.kernel.org/bpf/2e267b4b-0540-45d8-9310-e127bf95fc63@nbd.name/
Fixes: 6ebc5030e0c5 ("bpf: Fix array bounds error with may_goto")
Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
---
changes:
v3:
- Remove the selftest (Puranjay and Eduard)

v2:
- Addressed comments from Alexei
- Add selftest
 https://lore.kernel.org/all/20250813152958.3107403-1-kafai.wan@linux.dev/

v1:
 https://lore.kernel.org/all/20250805115513.4018532-1-kafai.wan@linux.dev/
---
 kernel/bpf/core.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index f8ac77d08ca7..e4568d44e827 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -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
-- 
2.43.0