[PATCH v4 3/5] selftest/bpf: Fixing powerpc JIT disassembly failure

[adubey@linux.ibm.com]

From: Abhishek Dubey <adubey@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 (for both 64-bit and 32-bit PowerPC)
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 bytes from JITed buffers
before supplying them for disassembly.

Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
---
 tools/testing/selftests/bpf/jit_disasm_helpers.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/jit_disasm_helpers.c b/tools/testing/selftests/bpf/jit_disasm_helpers.c
index 364c557c5115..4c6bcbe08491 100644
--- a/tools/testing/selftests/bpf/jit_disasm_helpers.c
+++ b/tools/testing/selftests/bpf/jit_disasm_helpers.c
@@ -170,9 +170,11 @@ 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;
 	__u32 *func_lens = NULL;
 	FILE *text_out = NULL;
 	uint8_t *image = NULL;
+	char *triple = NULL;
 	int i, err = 0;
 
 	if (!llvm_initialized) {
@@ -216,9 +218,18 @@ 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 for 64-bit and 32-bit powerpc, which can't
+	 * disassemble a to valid instruction.
+	 */
+	triple = LLVMGetDefaultTargetTriple();
+	if (strstr(triple, "powerpc"))
+		trunc_len = 8;
+
 	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]);
+		disasm_one_func(text_out, image + pc, func_lens[i] - trunc_len);
 		fprintf(text_out, "\n");
 		pc += func_lens[i];
 	}
-- 
2.52.0