[binutils-gdb] [gdb/tdep] Fix gdb.base/siginfo.exp on s390x-linux
Tom de Vries
vries@sourceware.org
Sat Jan 17 07:45:01 GMT 2026
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=8fa65585b772775e8ffe14bbfa38917c5a804cac
commit 8fa65585b772775e8ffe14bbfa38917c5a804cac
Author: Tom de Vries <tdevries@suse.de>
Date: Sat Jan 17 08:44:57 2026 +0100
[gdb/tdep] Fix gdb.base/siginfo.exp on s390x-linux
On s390x-linux (SLES 15 SP5), I'm running into:
...
FAIL: gdb.base/siginfo.exp: backtrace for nexti (pattern 2)
FAIL: gdb.base/siginfo.exp: step out of handler
...
The first FAIL is caused by a failure to unwind:
...
(gdb) bt^M
#0 handler (sig=26, info=0x3ffffffe428, context=0x3ffffffe4a8) at \
gdb.base/siginfo.c:31^M
Backtrace stopped: Cannot access memory at address 0x1a00000088^M
(gdb)
...
In contrast, on x86_64-linux I get instead:
...
(gdb) bt^M
#0 handler (sig=26, info=0x7fffffffc170, context=0x7fffffffc040) at \
gdb.base/siginfo.c:31^M
#1 <signal handler called>^M
#2 0x0000000000401201 in main () at gdb.base/siginfo.c:67^M
(gdb)
...
The memory access error is triggered here in s390_sigtramp_frame_unwind_cache:
...
/* Restore the previous frame's SP. */
prev_sp = read_memory_unsigned_integer (
info->saved_regs[S390_SP_REGNUM].addr (),
word_size, byte_order);
...
while trying to read an "Old-style RT frame" (for syscall sigreturn).
The problem is that we actually have a "New-style RT frame" (for syscall
rt_sigreturn).
[ See linux kernel source file arch/s390/kernel/signal.c for a detailed
explanation of the two. ]
The choice between the two is made earlier in that same function:
...
/* New-style RT frame:
retcode + alignment (8 bytes)
siginfo (128 bytes)
ucontext (contains sigregs at offset 5 words). */
if (next_ra == next_cfa)
{
...
}
/* Old-style RT frame and all non-RT frames:
old signal mask (8 bytes)
pointer to sigregs. */
else
...
I'm not sure why the check gives the wrong result, but I noticed that
s390_sigtramp_frame_sniffer is able to distinguish between the two, so fix
this by:
- factoring out new function s390_sigtramp_p out of
s390_sigtramp_frame_sniffer, and
- using s390_sigtramp_p in s390_sigtramp_frame_unwind_cache to distinguish
between the "Old-style RT frame" and "New-style RT frame".
This fixes the backtrace.
The second failure is:
...
(gdb) step^M
32 } /* handler */^M
1: x/i $pc^M
=> 0x1000772 <handler+50>: nopr^M
(gdb) step^M
0x000003fffdffe490 in __kernel_rt_sigreturn ()^M
1: x/i $pc^M
=> 0x3fffdffe490 <__kernel_rt_sigreturn>: svc 173^M
(gdb) FAIL: gdb.base/siginfo.exp: step out of handler
...
There is some code in process_event_stop_test that is supposed to trigger:
...
if (ecs->event_thread->control.step_range_end != 1
&& (ecs->event_thread->control.step_over_calls == STEP_OVER_UNDEBUGGABLE
|| ecs->event_thread->control.step_over_calls == STEP_OVER_ALL)
&& get_frame_type (frame) == SIGTRAMP_FRAME)
{
infrun_debug_printf ("stepped into signal trampoline");
/* The inferior, while doing a "step" or "next", has ended up in
a signal trampoline (either by a signal being delivered or by
the signal handler returning). Just single-step until the
inferior leaves the trampoline (either by calling the handler
or returning). */
keep_going (ecs);
return;
}
...
but it doesn't because frame is a NORMAL_FRAME instead of a SIGTRAMP_FRAME.
This is caused by the "dwarf2" unwinder triggering, which has higher priority
than the "s390 linux sigtramp" unwinder:
...
(gdb) maint info frame-unwinders
Name Type Class Enabled
dummy DUMMY_FRAME GDB Y
dwarf2 tailcall TAILCALL_FRAME DEBUGINFO Y
inline INLINE_FRAME GDB Y
jit NORMAL_FRAME EXTENSION Y
python NORMAL_FRAME EXTENSION Y
dwarf2 NORMAL_FRAME DEBUGINFO Y
dwarf2 signal SIGTRAMP_FRAME DEBUGINFO Y
s390 linux sigtramp SIGTRAMP_FRAME ARCH Y
s390 stub NORMAL_FRAME ARCH Y
s390 prologue NORMAL_FRAME ARCH Y
...
I found some code in dwarf2_frame_sniffer:
...
/* On some targets, signal trampolines may have unwind information.
We need to recognize them so that we set the frame type
correctly. */
if (fde->cie->signal_frame
|| dwarf2_frame_signal_frame_p (get_frame_arch (this_frame),
this_frame))
return self->type () == SIGTRAMP_FRAME;
...
and an example implementation i386_linux_dwarf_signal_frame_p, and after
copying this approach, indeed the stepping failure was fixed, but the
backtrace broken again.
Instead, fix this by giving the "s390 linux sigtramp" unwinder a higher
priority:
...
(gdb) maint info frame-unwinders
Name Type Class Enabled
dummy DUMMY_FRAME GDB Y
dwarf2 tailcall TAILCALL_FRAME DEBUGINFO Y
inline INLINE_FRAME GDB Y
jit NORMAL_FRAME EXTENSION Y
python NORMAL_FRAME EXTENSION Y
s390 linux sigtramp SIGTRAMP_FRAME ARCH Y
dwarf2 NORMAL_FRAME DEBUGINFO Y
dwarf2 signal SIGTRAMP_FRAME DEBUGINFO Y
s390 stub NORMAL_FRAME ARCH Y
s390 prologue NORMAL_FRAME ARCH Y
...
Also fixes test-case gdb.base/sigaltstack.exp and gdb.base/sigbpt.exp.
Tested on s390x-linux.
Reviewed-By: Keith Seitz <keiths@redhat.com>
PR tdep/33708
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33708
Diff:
---
gdb/s390-linux-tdep.c | 50 ++++++++++++++++++++++++++++++++------------------
1 file changed, 32 insertions(+), 18 deletions(-)
diff --git a/gdb/s390-linux-tdep.c b/gdb/s390-linux-tdep.c
index 3106a078621..f8ec7c99463 100644
--- a/gdb/s390-linux-tdep.c
+++ b/gdb/s390-linux-tdep.c
@@ -385,6 +385,30 @@ struct s390_sigtramp_unwind_cache {
trad_frame_saved_reg *saved_regs;
};
+/* Return true if the frame pc of THIS_FRAME points to either
+ __kernel_rt_sigreturn or __kernel_sigreturn. Return the corresponding
+ syscall number in SYSCALL_NR. */
+
+static bool
+s390_sigtramp_p (const frame_info_ptr &this_frame, int *syscall_nr_ptr)
+{
+ CORE_ADDR pc = get_frame_pc (this_frame);
+ bfd_byte sigreturn[2];
+
+ if (target_read_memory (pc, sigreturn, 2))
+ return false;
+
+ if (sigreturn[0] != op_svc)
+ return false;
+
+ int syscall_nr = sigreturn[1];
+ if (syscall_nr_ptr != nullptr)
+ *syscall_nr_ptr = syscall_nr;
+
+ return (syscall_nr == 119 /* sigreturn */
+ || syscall_nr == 173 /* rt_sigreturn */);
+}
+
/* Unwind THIS_FRAME and return the corresponding unwind cache for
s390_sigtramp_frame_unwind. */
@@ -397,7 +421,7 @@ s390_sigtramp_frame_unwind_cache (const frame_info_ptr &this_frame,
int word_size = gdbarch_ptr_bit (gdbarch) / 8;
enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
ULONGEST this_sp, prev_sp;
- CORE_ADDR next_ra, next_cfa, sigreg_ptr, sigreg_high_off;
+ CORE_ADDR next_cfa, sigreg_ptr, sigreg_high_off;
int i;
if (*this_prologue_cache)
@@ -408,14 +432,17 @@ s390_sigtramp_frame_unwind_cache (const frame_info_ptr &this_frame,
info->saved_regs = trad_frame_alloc_saved_regs (this_frame);
this_sp = get_frame_register_unsigned (this_frame, S390_SP_REGNUM);
- next_ra = get_frame_pc (this_frame);
next_cfa = this_sp + 16*word_size + 32;
+ int syscall_nr;
+ bool res = s390_sigtramp_p (this_frame, &syscall_nr);
+ gdb_assert (res);
+
/* New-style RT frame:
retcode + alignment (8 bytes)
siginfo (128 bytes)
ucontext (contains sigregs at offset 5 words). */
- if (next_ra == next_cfa)
+ if (syscall_nr == 173 /* rt_sigreturn */)
{
sigreg_ptr = next_cfa + 8 + 128 + align_up (5*word_size, 8);
/* sigregs are followed by uc_sigmask (8 bytes), then by the
@@ -525,20 +552,7 @@ s390_sigtramp_frame_sniffer (const struct frame_unwind *self,
const frame_info_ptr &this_frame,
void **this_prologue_cache)
{
- CORE_ADDR pc = get_frame_pc (this_frame);
- bfd_byte sigreturn[2];
-
- if (target_read_memory (pc, sigreturn, 2))
- return 0;
-
- if (sigreturn[0] != op_svc)
- return 0;
-
- if (sigreturn[1] != 119 /* sigreturn */
- && sigreturn[1] != 173 /* rt_sigreturn */)
- return 0;
-
- return 1;
+ return s390_sigtramp_p (this_frame, nullptr) ? 1 : 0;
}
/* S390 sigtramp frame unwinder. */
@@ -1187,7 +1201,7 @@ s390_linux_init_abi_any (struct gdbarch_info info, struct gdbarch *gdbarch)
set_gdbarch_get_syscall_number (gdbarch, s390_linux_get_syscall_number);
/* Frame handling. */
- frame_unwind_append_unwinder (gdbarch, &s390_sigtramp_frame_unwind);
+ frame_unwind_prepend_unwinder (gdbarch, &s390_sigtramp_frame_unwind);
set_gdbarch_skip_trampoline_code (gdbarch, find_solib_trampoline_target);
/* Enable TLS support. */
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