Re: [PATCH v10 3/8] selftest/bpf: Fixing powerpc JIT disassembly failure

[Hari Bathini]

On 06/08/26 10:49 pm, Hari Bathini wrote:

On 05/08/26 11:36 am, Saket Kumar Bhaskar wrote:

From: Abhishek Dubey <adu...@linux.ibm.com>

Ensure that the trampoline stubs JITed at the tail of the
epilogue do not expose the dummy trampoline address stored
in the last 8 bytes(64-bit) and last 4 bytes(32-bit)
to the disassembly flow. Prevent the disassembler from
ingesting this memory address, as it may occasionally decode
into a seemingly valid but incorrect instruction. Fix this
issue by truncating the last 8/4 bytes from JITed buffers
before supplying them for disassembly.

Fixes: b991fc520700 ("selftests/bpf: utility function to get program disassembly after jit")

The fixes tag on this patch doesn't really help. Please drop it.

With that..

Reviewed-by: Hari Bathini <hbath...@linux.ibm.com>

Signed-off-by: Abhishek Dubey <adu...@linux.ibm.com>
Signed-off-by: Saket Kumar Bhaskar <sk...@linux.ibm.com>
Tested-by: Yeswanth Krishna Tellakula <yeswa...@linux.ibm.com>
---
  .../selftests/bpf/jit_disasm_helpers.c        | 25 ++++++++++++++++++-
  1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/jit_disasm_helpers.c b/tools/ testing/selftests/bpf/jit_disasm_helpers.c

index 3558fe10e28c..c4aa1d69d3bb 100644
--- a/tools/testing/selftests/bpf/jit_disasm_helpers.c
+++ b/tools/testing/selftests/bpf/jit_disasm_helpers.c

@@ -179,9 +179,11 @@ int get_jited_program_text(int fd, char *text, size_t text_sz)

      struct bpf_prog_info info = {};
      __u32 info_len = sizeof(info);
      __u32 jited_funcs, len, pc;
+    __u32 trunc_len = 0, disasm_len;
      __u32 *func_lens = NULL;
      FILE *text_out = NULL;
      uint8_t *image = NULL;
+    char *triple = NULL;
      int i, err = 0;
      if (!llvm_initialized) {

@@ -225,9 +227,30 @@ int get_jited_program_text(int fd, char *text, size_t text_sz)

      if (!ASSERT_OK(err, "bpf_prog_get_info_by_fd #2"))
          goto out;
+    /*
+     * last 8 bytes contains dummy_trampoline address in JIT
+     * output on 64-bit and last 4 bytes on 32-bit powerpc,
+     * which can't disassemble to a valid instruction.
+     */
+    triple = LLVMGetDefaultTargetTriple();
+    if (triple) {
+        if (strstr(triple, "powerpc64") || strstr(triple, "ppc64"))
+            trunc_len = 8;
+        else if (strstr(triple, "powerpc") || strstr(triple, "ppc"))
+            trunc_len = 4;
+        LLVMDisposeMessage(triple);
+    }
+
      for (pc = 0, i = 0; i < jited_funcs; ++i) {
+
          fprintf(text_out, "func #%d:\n", i);
-        disasm_one_func(text_out, image + pc, func_lens[i]);
+        /*
+         * Disabled JIT have zero func_lens, hence underflow
+         */
+        disasm_len = func_lens[i] > trunc_len ?
+                    func_lens[i] - trunc_len : 0;
+        disasm_one_func(text_out, image + pc, disasm_len);
+
          fprintf(text_out, "\n");
          pc += func_lens[i];
      }

- Hari