Subject: [PATCH bpf-next 6/6] bpf, verifier: always mark destination of LDX as 64-bit

From: Puranjay Mohan <[email-masked]>
Date: Tue, 12 Sep 2023 22:46:54 +0000

All 64-bit JITs utilize a single instruction to load + zero-extend a
byte, word, or a half-word. The optimisation of emitting zext for LDX is
not useful for most of the JITs.

All the JITs that relied on the verifier for zero extension of LDX
desitination registers have been modified to always zero extend the
destination.

Now the verifier can safely mark LDX destination as 64-bit and stop
emitting zero-extension instructions for it.

Signed-off-by: Puranjay Mohan <[email-masked]>
---
 kernel/bpf/verifier.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index dbba2b806017..02a1ac1a1327 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3028,9 +3028,7 @@ static bool is_reg64(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		return false;
 
 	if (class == BPF_LDX) {
-		if (t != SRC_OP)
-			return BPF_SIZE(code) == BPF_DW;
-		/* LDX source must be ptr. */
+		/* LDX source must be a ptr. and LDX destination is always zero-extended. */
 		return true;
 	}
 
-- 
2.39.2