[PATCH v2 28/47] Windows gdb+gdbserver: Make siginfo_er per-thread state

Pedro Alves
2025-05-19T13:32:43Z

With non-stop mode support, each thread has its own "last event", and
so printing $_siginfo should print the siginfo of the selected thread.
Likewise, with all-stop and scheduler-locking.

This patch reworks the siginfo functions in gdb/windows-nat.c and
gdbserver/win32-low.cc to reuse the exception record already saved
within each thread's 'last_event' field.

Here's an example of what you'll see after the whole non-stop series:

(gdb) thread apply all p -pretty -- $_siginfo

Thread 3 (Thread 2612.0x1470):
$1 = {
ExceptionCode = DBG_CONTROL_C,
ExceptionFlags = 0,
ExceptionRecord = 0x0,
ExceptionAddress = 0x7ffd0583e929 <KERNELBASE!EncodeRemotePointer+8249>,
NumberParameters = 0,
{
ExceptionInformation = {0 <repeats 15 times>},
AccessViolationInformation = {
Type = READ_ACCESS_VIOLATION,
Address = 0x0
}
}
}

Thread 2 (Thread 2612.0x1704):
$2 = {
ExceptionCode = SINGLE_STEP,
ExceptionFlags = 0,
ExceptionRecord = 0x0,
ExceptionAddress = 0x7ffd080ad6e4 <ntdll!ZwDelayExecution+20>,
NumberParameters = 0,
{
ExceptionInformation = {0 <repeats 15 times>},
AccessViolationInformation = {
Type = READ_ACCESS_VIOLATION,
Address = 0x0
}
}
}

Thread 1 (Thread 2612.0x434):
$3 = {
ExceptionCode = BREAKPOINT,
ExceptionFlags = 0,
ExceptionRecord = 0x0,
ExceptionAddress = 0x7ff6f691174c <main+185>,
NumberParameters = 1,
{
ExceptionInformation = {0 <repeats 15 times>},
AccessViolationInformation = {
Type = READ_ACCESS_VIOLATION,
Address = 0x0
}
}
}
(gdb)

This was in non-stop mode, and the program originally had two threads.
Thread 1 stopped for a breakpoint, then thread 2 was manually
interrupted/paused and then single-stepped. And then I typed Ctrl-C
in the inferior's terminal, which made Windows inject thread 3 in the
inferior, and report a DBG_CONTROL_C exception for it.

Change-Id: I5d4f1b62f59e8aef3606642c6524df2362b0fb7d
---
gdb/nat/windows-nat.c | 2 --
gdb/nat/windows-nat.h | 3 ---
gdb/windows-nat.c | 33 ++++++++++++++++-----------------
gdbserver/win32-low.cc | 27 ++++++++++++++-------------
4 files changed, 30 insertions(+), 35 deletions(-)

diff --git a/gdb/nat/windows-nat.c b/gdb/nat/windows-nat.c
index 864788ec125..6128b83bea4 100644
--- a/gdb/nat/windows-nat.c
+++ b/gdb/nat/windows-nat.c
@@ -357,8 +357,6 @@ windows_process_info::handle_exception (DEBUG_EVENT &current_event,
DWORD code = rec->ExceptionCode;
handle_exception_result result = HANDLE_EXCEPTION_HANDLED;

- memcpy (&siginfo_er, rec, sizeof siginfo_er);
-
switch (code)
{
case EXCEPTION_ACCESS_VIOLATION:
diff --git a/gdb/nat/windows-nat.h b/gdb/nat/windows-nat.h
index 71cb118f5d6..8da9ca6e78f 100644
--- a/gdb/nat/windows-nat.h
+++ b/gdb/nat/windows-nat.h
@@ -152,9 +152,6 @@ struct windows_process_info
DWORD process_id = 0;
DWORD main_thread_id = 0;

- /* Contents of $_siginfo */
- EXCEPTION_RECORD siginfo_er {};
-
#ifdef __x86_64__
/* The target is a WOW64 process */
bool wow64_process = false;
diff --git a/gdb/windows-nat.c b/gdb/windows-nat.c
index 87cf5e1a4e8..09c2d224fd0 100644
--- a/gdb/windows-nat.c
+++ b/gdb/windows-nat.c
@@ -2888,7 +2888,6 @@ windows_nat_target::mourn_inferior ()
CHECK (CloseHandle (windows_process.handle));
windows_process.open_process_used = 0;
}
- windows_process.siginfo_er.ExceptionCode = 0;
inf_child_target::mourn_inferior ();
}

@@ -3005,8 +3004,15 @@ static enum target_xfer_status
windows_xfer_siginfo (gdb_byte *readbuf, ULONGEST offset, ULONGEST len,
ULONGEST *xfered_len)
{
- char *buf = (char *) &windows_process.siginfo_er;
- size_t bufsize = sizeof (windows_process.siginfo_er);
+ windows_thread_info *th = windows_process.find_thread (inferior_ptid);
+
+ if (th->last_event.dwDebugEventCode != EXCEPTION_DEBUG_EVENT)
+ return TARGET_XFER_E_IO;
+
+ EXCEPTION_RECORD &er = th->last_event.u.Exception.ExceptionRecord;
+
+ char *buf = (char *) &er;
+ size_t bufsize = sizeof (er);

#ifdef __x86_64__
EXCEPTION_RECORD32 er32;
@@ -3015,23 +3021,16 @@ windows_xfer_siginfo (gdb_byte *readbuf, ULONGEST offset, ULONGEST len,
buf = (char *) &er32;
bufsize = sizeof (er32);

- er32.ExceptionCode = windows_process.siginfo_er.ExceptionCode;
- er32.ExceptionFlags = windows_process.siginfo_er.ExceptionFlags;
- er32.ExceptionRecord
- = (uintptr_t) windows_process.siginfo_er.ExceptionRecord;
- er32.ExceptionAddress
- = (uintptr_t) windows_process.siginfo_er.ExceptionAddress;
- er32.NumberParameters = windows_process.siginfo_er.NumberParameters;
- int i;
- for (i = 0; i < EXCEPTION_MAXIMUM_PARAMETERS; i++)
- er32.ExceptionInformation[i]
- = windows_process.siginfo_er.ExceptionInformation[i];
+ er32.ExceptionCode = er.ExceptionCode;
+ er32.ExceptionFlags = er.ExceptionFlags;
+ er32.ExceptionRecord = (uintptr_t) er.ExceptionRecord;
+ er32.ExceptionAddress = (uintptr_t) er.ExceptionAddress;
+ er32.NumberParameters = er.NumberParameters;
+ for (int i = 0; i < EXCEPTION_MAXIMUM_PARAMETERS; i++)
+ er32.ExceptionInformation[i] = er.ExceptionInformation[i];
}
#endif

- if (windows_process.siginfo_er.ExceptionCode == 0)
- return TARGET_XFER_E_IO;
-
if (readbuf == nullptr)
return TARGET_XFER_E_IO;

diff --git a/gdbserver/win32-low.cc b/gdbserver/win32-low.cc
index c3e35a6f602..c95ec112bf7 100644
--- a/gdbserver/win32-low.cc
+++ b/gdbserver/win32-low.cc
@@ -654,7 +654,6 @@ win32_clear_process ()
}

for_each_thread (delete_thread_info);
- windows_process.siginfo_er.ExceptionCode = 0;
}

/* Implementation of target_ops::kill. */
@@ -1284,14 +1283,18 @@ win32_process_target::qxfer_siginfo (const char *annex,
unsigned const char *writebuf,
CORE_ADDR offset, int len)
{
- if (windows_process.siginfo_er.ExceptionCode == 0)
+ windows_thread_info *th = windows_process.find_thread (current_thread->id);
+
+ if (th->last_event.dwDebugEventCode != EXCEPTION_DEBUG_EVENT)
return -1;

if (readbuf == nullptr)
return -1;

- char *buf = (char *) &windows_process.siginfo_er;
- size_t bufsize = sizeof (windows_process.siginfo_er);
+ EXCEPTION_RECORD &er = th->last_event.u.Exception.ExceptionRecord;
+
+ char *buf = (char *) &er;
+ size_t bufsize = sizeof (er);

#ifdef __x86_64__
EXCEPTION_RECORD32 er32;
@@ -1300,17 +1303,15 @@ win32_process_target::qxfer_siginfo (const char *annex,
buf = (char *) &er32;
bufsize = sizeof (er32);

- er32.ExceptionCode = windows_process.siginfo_er.ExceptionCode;
- er32.ExceptionFlags = windows_process.siginfo_er.ExceptionFlags;
+ er32.ExceptionCode = er.ExceptionCode;
+ er32.ExceptionFlags = er.ExceptionFlags;
er32.ExceptionRecord
- = (uintptr_t) windows_process.siginfo_er.ExceptionRecord;
+ = (uintptr_t) er.ExceptionRecord;
er32.ExceptionAddress
- = (uintptr_t) windows_process.siginfo_er.ExceptionAddress;
- er32.NumberParameters = windows_process.siginfo_er.NumberParameters;
- int i;
- for (i = 0; i < EXCEPTION_MAXIMUM_PARAMETERS; i++)
- er32.ExceptionInformation[i]
- = windows_process.siginfo_er.ExceptionInformation[i];
+ = (uintptr_t) er.ExceptionAddress;
+ er32.NumberParameters = er.NumberParameters;
+ for (int i = 0; i < EXCEPTION_MAXIMUM_PARAMETERS; i++)
+ er32.ExceptionInformation[i] = er.ExceptionInformation[i];
}
#endif

--
2.49.0