[PATCH RFC] bpf, x64: allow not-converged images when BPF_JIT_ALWAYS_ON is set

[Gary Lin <glin@suse.com>]

The x64 bpf jit expects the bpf images converge within the given passes.
However there is a corner case:

  l0:     ldh [4]
  l1:     jeq #0x537d, l2, l40
  l2:     ld [0]
  l3:     jeq #0xfa163e0d, l4, l40
  l4:     ldh [12]
  l5:     ldx #0xe
  l6:     jeq #0x86dd, l41, l7
  l7:     jeq #0x800, l8, l41
  l8:     ld [x+16]
  l9:     ja 41

    [... repeated ja 41 ]

  l40:    ja 41
  l41:    ret #0
  l42:    ld #len
  l43:    ret a

The bpf program contains 32 "ja 41" and do_jit() only removes one "ja 41"
right before "l41:  ret #0" for offset==0 in each pass, so
bpf_int_jit_compile() needs to run do_jit() at least 32 times to
eliminate those JMP instructions. Since the current max number of passes
is 20, the bpf program couldn't converge within 20 passes and got rejected
when BPF_JIT_ALWAYS_ON is set even though it's legit as a classic socket
filter.

A not-converged image may be not optimal but at least the bpf
instructions are translated into x64 machine code. Maybe we could just
issue a warning instead so that the program is still loaded and the user
is also notified.

On the other hand, if the size convergence is mandatory, then it
deserves a document to collect the corner cases so that the user could
know the limitations and how to work around them.

Signed-off-by: Gary Lin <glin@suse.com>
---
 arch/x86/net/bpf_jit_comp.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 796506dcfc42..90814c2daaae 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1972,6 +1972,8 @@ struct x64_jit_data {
 	struct jit_context ctx;
 };
 
+#define MAX_JIT_PASSES 20
+
 struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 {
 	struct bpf_binary_header *header = NULL;
@@ -2042,7 +2044,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 	 * may converge on the last pass. In such case do one more
 	 * pass to emit the final image.
 	 */
-	for (pass = 0; pass < 20 || image; pass++) {
+	for (pass = 0; pass < MAX_JIT_PASSES || image; pass++) {
 		proglen = do_jit(prog, addrs, image, oldproglen, &ctx);
 		if (proglen <= 0) {
 out_image:
@@ -2054,13 +2056,22 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 		}
 		if (image) {
 			if (proglen != oldproglen) {
+#ifdef CONFIG_BPF_JIT_ALWAYS_ON
+				pr_warn("bpf_jit: proglen=%d != oldproglen=%d pass=%d\n",
+					proglen, oldproglen, pass);
+#else
 				pr_err("bpf_jit: proglen=%d != oldproglen=%d\n",
 				       proglen, oldproglen);
 				goto out_image;
+#endif
 			}
 			break;
 		}
+#ifdef CONFIG_BPF_JIT_ALWAYS_ON
+		if (proglen == oldproglen || pass >= (MAX_JIT_PASSES - 1)) {
+#else
 		if (proglen == oldproglen) {
+#endif
 			/*
 			 * The number of entries in extable is the number of BPF_LDX
 			 * insns that access kernel memory via "pointer to BTF type".
-- 
2.28.0