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 -i mon1 \
"not wlan addr3 3c37121a2b3c and not wlan addr2 184ecbca2a3a \
and not wlan addr2 14130b4d3f47 and not wlan addr2 f0f61cf440b7 \
and not wlan addr3 a84b4dedf471 and not wlan addr3 d022be17e1d7 \
and not wlan addr3 5c497967208b and not wlan addr2 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 <[email protected]>
Closes: https://lore.kernel.org/bpf/[email protected]/
Fixes: 6ebc503 ("bpf: Fix array bounds error with may_goto")
Fixes: 86bc9c7 ("bpf: Avoid __bpf_prog_ret0_warn when jit fails")
Signed-off-by: KaFai Wan <[email protected]>

Author: mannkafai

kernel/bpf/core.c

@@ -2366,8 +2366,8 @@ 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!
+	 * or may_goto may cause stack size > 512 is not working properly,
+	 * so warn about it!
 	 */
 	WARN_ON_ONCE(1);
 	return 0;
@@ -2478,10 +2478,10 @@ 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];
 	} else {
+		WARN_ON_ONCE(!fp->jit_requested);
 		fp->bpf_func = __bpf_prog_ret0_warn;
 	}
 #else
@@ -2505,7 +2505,7 @@ 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;
@@ -2515,6 +2515,8 @@ struct bpf_prog *bpf_prog_select_runtime(struct bpf_prog *fp, int *err)
 		jit_needed = true;
 
 	bpf_prog_select_func(fp);
+	if (fp->bpf_func == __bpf_prog_ret0_warn)
+		jit_needed = true;
 
 	/* eBPF JITs can rewrite the program in case constant
 	 * blinding is active. However, in case of error during