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Use RAII to set the per-thread SIGSEGV handler

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This avoids using a thread-local extern variable, which causes link errors
on some platforms, notably Cygwin.  But I think this is a better pattern
even outside of working around linker bugs because it encapsulates direct
access to the variable inside the class, instead of having a global extern
variable.

The cygwin link error is:
cp-support.o: in function `gdb_demangle(char const*, int)':
/home/Christian/binutils-gdb/obj/gdb/../../gdb/cp-support.c:1619:(.text+0x6472): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `TLS init function for thread_local_segv_handler'
/home/Christian/binutils-gdb/obj/gdb/../../gdb/cp-support.c:1619:(.text+0x648b): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `TLS init function for thread_local_segv_handler'
collect2: error: ld returned 1 exit status

2021-03-12  Christian Biesinger  <cbiesinger@google.com>

	PR threads/27239
	* cp-support.c: Use scoped_segv_handler_restore.
	* event-top.c (thread_local_segv_handler): Made static.
	(scoped_segv_handler_restore::scoped_segv_handler_restore):
	New function.
	(scoped_segv_handler_restore::~scoped_segv_handler_restore): New
	function.
	* event-top.h (class scoped_segv_handler_restore): New class.
	(thread_local_segv_handler): Removed.
This commit is contained in:
Christian Biesinger
2021-03-09 08:16:23 -06:00
parent 354d37286a
commit 1ee63759b0
4 changed files with 51 additions and 12 deletions
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12
gdb/ChangeLog
View File

@@ -1,3 +1,15 @@
2021-03-12 Christian Biesinger <cbiesinger@google.com>
PR threads/27239
* cp-support.c: Use scoped_segv_handler_restore.
* event-top.c (thread_local_segv_handler): Made static.
(scoped_segv_handler_restore::scoped_segv_handler_restore):
New function.
(scoped_segv_handler_restore::~scoped_segv_handler_restore): New
function.
* event-top.h (class scoped_segv_handler_restore): New class.
(thread_local_segv_handler): Removed.
2021-03-07 Tom de Vries <tdevries@suse.de>
PR symtab/27341

9
gdb/cp-support.c
View File

@@ -1615,11 +1615,10 @@ gdb_demangle (const char *name, int options)
int crash_signal = 0;
#ifdef HAVE_WORKING_FORK
scoped_restore restore_segv
= make_scoped_restore (&thread_local_segv_handler,
catch_demangler_crashes
? gdb_demangle_signal_handler
: nullptr);
scoped_segv_handler_restore restore_segv
(catch_demangler_crashes
? gdb_demangle_signal_handler
: nullptr);
bool core_dump_allowed = gdb_demangle_attempt_core_dump;
SIGJMP_BUF jmp_buf;

23
gdb/event-top.c
View File

@@ -849,9 +849,17 @@ gdb_readline_no_editing_callback (gdb_client_data client_data)
}
/* See event-top.h. */
/* The SIGSEGV handler for this thread, or NULL if there is none. GDB
always installs a global SIGSEGV handler, and then lets threads
indicate their interest in handling the signal by setting this
thread-local variable.
thread_local void (*thread_local_segv_handler) (int);
This is a static variable instead of extern because on various platforms
(notably Cygwin) extern thread_local variables cause link errors. So
instead, we have scoped_segv_handler_restore, which also makes it impossible
to accidentally forget to restore it to the original value. */
static thread_local void (*thread_local_segv_handler) (int);
static void handle_sigsegv (int sig);
@@ -1286,3 +1294,14 @@ gdb_disable_readline (void)
gdb_rl_callback_handler_remove ();
delete_file_handler (ui->input_fd);
}
scoped_segv_handler_restore::scoped_segv_handler_restore (segv_handler_t new_handler)
{
m_old_handler = thread_local_segv_handler;
thread_local_segv_handler = new_handler;
}
scoped_segv_handler_restore::~scoped_segv_handler_restore()
{
thread_local_segv_handler = m_old_handler;
}

19
gdb/event-top.h
View File

@@ -70,10 +70,19 @@ extern void gdb_rl_callback_handler_install (const char *prompt);
currently installed. */
extern void gdb_rl_callback_handler_reinstall (void);
/* The SIGSEGV handler for this thread, or NULL if there is none. GDB
always installs a global SIGSEGV handler, and then lets threads
indicate their interest in handling the signal by setting this
thread-local variable. */
extern thread_local void (*thread_local_segv_handler) (int);
typedef void (*segv_handler_t) (int);
/* On construction, replaces the current thread's SIGSEGV handler with
the provided one. On destruction, restores the handler to the
original one. */
class scoped_segv_handler_restore
{
public:
scoped_segv_handler_restore (segv_handler_t new_handler);
~scoped_segv_handler_restore ();
private:
segv_handler_t m_old_handler;
};
#endif