[PATCH v2 2/2] Refactor amd64_analyze_prologue

Victor Collod
2020-06-11T22:56:42Z

* merge op and the buf array, which were both used for storing code
* invert conditions to avoid long nested ifs
* use target_read_code instead of read_code to gracefully handle errors
* `if (current_pc <= pc)' felt backwards, as current_pc doesn't change,
and the test could be described as "stop if pc went past current_pc"

2020-06-11 Victor Collod <vcollod@nvidia.com>

* amd64-tdep.c (amd64_analyze_prologue): Make the function more readable.
---
gdb/amd64-tdep.c | 93 ++++++++++++++++++++++++++----------------------
1 file changed, 50 insertions(+), 43 deletions(-)

diff --git a/gdb/amd64-tdep.c b/gdb/amd64-tdep.c
index 0ce9fbc2997..6c1a4a138de 100644
--- a/gdb/amd64-tdep.c
+++ b/gdb/amd64-tdep.c
@@ -2374,7 +2374,6 @@ amd64_analyze_prologue (struct gdbarch *gdbarch,
CORE_ADDR pc, CORE_ADDR current_pc,
struct amd64_frame_cache *cache)
{
- enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
/* The `endbr64` instruction. */
static const gdb_byte endbr64[4] = { 0xf3, 0x0f, 0x1e, 0xfa };
/* There are two variations of movq %rsp, %rbp. */
@@ -2384,10 +2383,10 @@ amd64_analyze_prologue (struct gdbarch *gdbarch,
static const gdb_byte mov_esp_ebp_1[2] = { 0x89, 0xe5 };
static const gdb_byte mov_esp_ebp_2[2] = { 0x8b, 0xec };

- gdb_byte buf[3];
- gdb_byte op;
+ gdb_byte buf[4];

- if (current_pc <= pc)
+ /* Analysis must not go past current_pc */
+ if (pc >= current_pc)
return current_pc;

if (gdbarch_ptr_bit (gdbarch) == 32)
@@ -2395,57 +2394,65 @@ amd64_analyze_prologue (struct gdbarch *gdbarch,
else
pc = amd64_analyze_stack_align (pc, current_pc, cache);

- op = read_code_unsigned_integer (pc, 1, byte_order);
-
- /* Check for the `endbr64` instruction, skip it if found. */
- if (op == endbr64[0])
+ /* Try to read enough bytes to check for `endbr64' */
+ if (target_read_code (pc, buf, 4) != 0)
+ {
+ /* If it fails, read just enough data for `pushq %rbp' */
+ if (target_read_code (pc, buf, 1) != 0)
+ return pc;
+ }
+ /* If reading succeeded, check for the `endbr64' instruction and skip it if found. */
+ else if (memcmp (buf, endbr64, sizeof (endbr64)) == 0)
{
- read_code (pc + 1, buf, 3);
+ pc += sizeof (endbr64);
+ /* If we went past the allowed bound, stop */
+ if (pc >= current_pc)
+ return current_pc;
+ /* update the code buffer, as pc changed */
+ if (target_read_code (pc, buf, 1) != 0)
+ return pc;
+ }

- if (memcmp (buf, &endbr64[1], 3) == 0)
- pc += 4;
+ /* stop right now if there's no `pushq %rbp' */
+ if (buf[0] != 0x55)
+ return pc;

- op = read_code_unsigned_integer (pc, 1, byte_order);
- }
+ /* Take into account that we've executed the `pushq %rbp' that
+ starts this instruction sequence. */
+ cache->saved_regs[AMD64_RBP_REGNUM] = 0;
+ cache->sp_offset += 8;

- if (current_pc <= pc)
- return current_pc;
+ pc += 1;

- if (op == 0x55) /* pushq %rbp */
- {
- /* Take into account that we've executed the `pushq %rbp' that
- starts this instruction sequence. */
- cache->saved_regs[AMD64_RBP_REGNUM] = 0;
- cache->sp_offset += 8;
+ /* If we went past the allowed bound, stop */
+ if (pc >= current_pc)
+ return current_pc;

- /* If that's all, return now. */
- if (current_pc <= pc + 1)
- return current_pc;
+ /* Try to read the code for the stack base move */
+ if (target_read_code (pc, buf, 3) != 0)
+ return pc;

- read_code (pc + 1, buf, 3);
+ /* Check for `movq %rsp, %rbp'. */
+ if (memcmp (buf, mov_rsp_rbp_1, 3) == 0
+ || memcmp (buf, mov_rsp_rbp_2, 3) == 0)
+ {
+ pc += 3;
+ /* OK, we actually have a frame. */
+ cache->frameless_p = 0;
+ return pc;
+ }

- /* Check for `movq %rsp, %rbp'. */
- if (memcmp (buf, mov_rsp_rbp_1, 3) == 0
- || memcmp (buf, mov_rsp_rbp_2, 3) == 0)
+ /* For X32, also check for `movl %esp, %ebp'. */
+ if (gdbarch_ptr_bit (gdbarch) == 32)
+ {
+ if (memcmp (buf, mov_esp_ebp_1, 2) == 0
+ || memcmp (buf, mov_esp_ebp_2, 2) == 0)
{
+ pc += 2;
/* OK, we actually have a frame. */
cache->frameless_p = 0;
- return pc + 4;
+ return pc;
}
-
- /* For X32, also check for `movq %esp, %ebp'. */
- if (gdbarch_ptr_bit (gdbarch) == 32)
- {
- if (memcmp (buf, mov_esp_ebp_1, 2) == 0
- || memcmp (buf, mov_esp_ebp_2, 2) == 0)
- {
- /* OK, we actually have a frame. */
- cache->frameless_p = 0;
- return pc + 3;
- }
- }
-
- return pc + 1;
}

return pc;
--
2.20.1