[PATCH] [gdb/tdep] Fix unrelocated pc in i386_displaced_step_fixup

Tom de Vries
2026-03-17T07:19:59Z

With test-case gdb.threads/next-fork-other-thread.exp and target board
unix/-m32 I run into:
...
(gdb) next^M
[Switching to Thread 0xf643ab40 (LWP 3267939)]^M
^M
Thread 5 "next-fork-other" hit Breakpoint 1, main () at \
next-fork-other-thread.c:73^M
73 alarm (60);^M
(gdb) FAIL: $exp: fork_func=fork: target-non-stop=off: non-stop=off: \
displaced-stepping=on: i=$n: next to break here
...

Before we go into how this happens, let's first look at the inferior.

In main, 4 threads are started with the same thread function, leaving all 5
threads in a loop:
- the main thread is stuck in a loop calling sleep, and gdb steps through this
loop using next
- the other, non-main threads are stuck in a loop where each thread:
- forks off a child process that exits immediately
- waits for the child process to exit
- calls sleep

The FAIL happens as follows (following snippets from this gdb.log [1]).

One of the non-main threads enters __syscall_cancel_arch to do a sycall (
either to sleep, or to wait).

Then the non-main thread stops:
...
[infrun] handle_signal_stop: [2937316.2937324.0] hit another thread's \
single-step breakpoint^M
[infrun] handle_signal_stop: delayed software breakpoint trap, ignoring^M
[infrun] switch_back_to_stepped_thread: need to step [2937316.2937324.0] \
over single-step breakpoint^M
...
because we "hit another thread's single-step breakpoint".

AFAIU, this is because of the main thread stepping through
__syscall_cancel_arch.

To handle this, we're going to try displaced stepping.

The syscall instruction is copied:
...
[displaced] displaced_step_prepare_throw: original insn 0xf7cdce69: \
cd 80 int $0x80^M
[displaced] prepare: selected buffer at 0x80490d2^M
...
to a buffer at _start+2:
...
080490d0 <_start>:
80490d0: 31 ed xor %ebp,%ebp
80490d2: 5e pop %esi
...
and we're going to resume execution there.

However, right after resuming we get a GDB_SIGNAL_CHLD for the same thread.

Part of handling that is finalizing the displaced stepping:
...
[displaced] finish: restored 2937316.2937324.0 0x80490d2^M
[displaced] i386_displaced_step_fixup: fixup (0xf7cdce69, 0x80490d2), \
insn = 0xcd 0x80 ...^M
[displaced] i386_displaced_step_fixup: syscall changed %eip; not relocating^M
[infrun] handle_signal_stop: stop_pc=0x80490d2^M
...

The stop pc is 0x80490d2, the address of the copied instruction. In other
words, we've stopped without making progress.

The problem is that the address is in the displaced stepping buffer, and needs
relocating, but instead we have "syscall changed %eip; not relocating".

The code in i386_displaced_step_fixup doesn't recognize this situation:
...
if (i386_syscall_p (insn, &insn_len)
&& pc != to + (insn - insn_start) + insn_len
/* GDB can get control back after the insn after the syscall.
Presumably this is a kernel bug.
i386_displaced_step_copy_insn ensures it's a nop,
we add one to the length for it. */
&& pc != to + (insn - insn_start) + insn_len + 1)
displaced_debug_printf ("syscall changed %%eip; not relocating");
...

It only handles the cases where the stop pc is:
- the address after the syscall insn, or
- the address after the nop after the syscall insn

So, instead of relocating the stop pc back to the original 0xf7cdce69, it
stays 0x80490d2. After resuming at that address, the thread:
- executes the syscall,
- executes the rest of _start,
- enters main, and
- runs into the breakpoint at the start of main.

Since commit cf141dd8ccd ("gdb: fix reg corruption from displaced stepping on
amd64"), we do handle the "pc == to" case in amd64_displaced_step_fixup:
...
if (amd64_syscall_p (insn_details, &insn_len)
/* GDB can get control back after the insn after the syscall.
Presumably this is a kernel bug. Fixup ensures its a nop, we
add one to the length for it. */
&& (pc < to || pc > (to + insn_len + 1)))
displaced_debug_printf ("syscall changed %%rip; not relocating");
...

Fix this in the same way.

Tested on x86_64-linux, with target board unix/-m32.

On openSUSE Tumbleweed (kernel version 6.19.7), this patch fixes the test-case.

On openSUSE Leap 16.0 (kernel version 6.12.0), we still run into PR29040.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33997

[1] https://sourceware.org/bugzilla/attachment.cgi?id=16660
---
gdb/i386-tdep.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/gdb/i386-tdep.c b/gdb/i386-tdep.c
index b9013e183c2..93357b41b10 100644
--- a/gdb/i386-tdep.c
+++ b/gdb/i386-tdep.c
@@ -813,12 +813,11 @@ i386_displaced_step_fixup (struct gdbarch *gdbarch,
it unrelocated. Goodness help us if there are PC-relative
system calls. */
if (i386_syscall_p (insn, &insn_len)
- && pc != to + (insn - insn_start) + insn_len
/* GDB can get control back after the insn after the syscall.
Presumably this is a kernel bug.
i386_displaced_step_copy_insn ensures it's a nop,
we add one to the length for it. */
- && pc != to + (insn - insn_start) + insn_len + 1)
+ && (pc < to || pc > to + (insn - insn_start) + insn_len + 1))
displaced_debug_printf ("syscall changed %%eip; not relocating");
else
{

base-commit: 9cc83ec0ce9b4b75e8cd2b0c46f23d4cbf4b2f2b
--
2.51.0