[PATCH v2 5/6] Support loading libunwind instead of libgcc_s
Adhemerval Zanella
adhemerval.zanella@linaro.org
Wed Feb 25 18:31:16 GMT 2026
The 'unwind-link' facility allows glibc to support thread cancellation
and exit (pthread_cancel, pthread_exiti, backtrace) by dynamically
loading the unwind library at runtime, preventing a hard dependency on
libgcc_s within libc.so.
When building with libunwind (for clang/LLVM toolchains), two assumptions
in the existing code break:
1. The runtime library is libunwind.so instead of libgcc_s.so.
2. libgcc relies on __gcc_personality_v0 to handle unwinding mechanics.
libunwind exposes the standard '_Unwind_*' accessors directly.
This patch adapts `unwind-link` to handle both environments based on
the HAVE_CC_WITH_LIBUNWIND configuration:
* The UNWIND_SONAME macro now selects between LIBGCC_S_SO and
LIBUNWIND_SO.
* For libgcc, it continues to resolve `__gcc_personality_v0`.
* For libunwind, it instead resolves the standard
_Unwind_GetLanguageSpecificData, _Unwind_SetGR, _Unwind_SetIP,
and _Unwind_GetRegionStart helpers.
* unwind-resume.c is updated to implement wrappers for these
accessors that forward calls to the dynamically loaded function
pointers, effectively shimming the unwinder.
Tests and Makefiles are updated to link against `$(libunwind)` where
appropriate.
---
elf/Makefile | 2 ++
misc/unwind-link.c | 34 ++++++++++++++++++++++++++++++++-
nptl/pthread_cancel.c | 2 +-
nptl/pthread_exit.c | 2 +-
shlib-versions | 2 ++
sysdeps/generic/unwind-link.h | 13 +++++++++++++
sysdeps/generic/unwind-resume.c | 32 +++++++++++++++++++++++++++++++
7 files changed, 84 insertions(+), 3 deletions(-)
diff --git a/elf/Makefile b/elf/Makefile
index bd7ae54632..889df3c12f 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -2763,6 +2763,8 @@ $(objpfx)tst-big-note-lib.so: $(objpfx)tst-big-note-lib.o
$(LINK.o) -shared -o $@ $(LDFLAGS.so) $(dt-relr-ldflag) $<
$(objpfx)tst-unwind-ctor: $(objpfx)tst-unwind-ctor-lib.so
+LDLIBS-tst-unwind-ctor += $(libunwind)
+LDFLAGS-tst-unwind-ctor-lib.so = -Wl,--unresolved-symbols=ignore-all
CFLAGS-tst-unwind-main.c += -funwind-tables -DUSE_PTHREADS=0
diff --git a/misc/unwind-link.c b/misc/unwind-link.c
index 6019dacf66..c5c1a3ea69 100644
--- a/misc/unwind-link.c
+++ b/misc/unwind-link.c
@@ -37,6 +37,12 @@ static void *global_libgcc_handle;
may depend on unwinding. */
__libc_lock_define (static, lock);
+#ifndef HAVE_CC_WITH_LIBUNWIND
+# define UNWIND_SONAME LIBGCC_S_SO
+#else
+# define UNWIND_SONAME LIBUNWIND_SO
+#endif
+
struct unwind_link *
__libc_unwind_link_get (void)
{
@@ -48,7 +54,7 @@ __libc_unwind_link_get (void)
/* Initialize a copy of the data, so that we do not need about
unlocking in case the dynamic loader somehow triggers
unwinding. */
- void *local_libgcc_handle = __libc_dlopen (LIBGCC_S_SO);
+ void *local_libgcc_handle = __libc_dlopen (UNWIND_SONAME);
if (local_libgcc_handle == NULL)
{
__libc_lock_unlock (lock);
@@ -72,8 +78,19 @@ __libc_unwind_link_get (void)
local.ptr___frame_state_for
= __libc_dlsym (local_libgcc_handle, "__frame_state_for");
#endif
+#ifndef HAVE_CC_WITH_LIBUNWIND
local.ptr_personality
= __libc_dlsym (local_libgcc_handle, "__gcc_personality_v0");
+#else
+ local.ptr__Unwind_GetLanguageSpecificData
+ = __libc_dlsym (local_libgcc_handle, "_Unwind_GetLanguageSpecificData");
+ local.ptr__Unwind_SetGR
+ = __libc_dlsym (local_libgcc_handle, "_Unwind_SetGR");
+ local.ptr__Unwind_SetIP
+ = __libc_dlsym (local_libgcc_handle, "_Unwind_SetIP");
+ local.ptr__Unwind_GetRegionStart
+ = __libc_dlsym (local_libgcc_handle, "_Unwind_GetRegionStart");
+#endif
UNWIND_LINK_EXTRA_INIT
/* If a symbol is missing, libgcc_s has somehow been corrupted. */
@@ -84,7 +101,15 @@ __libc_unwind_link_get (void)
assert (local.ptr__Unwind_GetIP != NULL);
#endif
assert (local.ptr__Unwind_Resume != NULL);
+#ifndef HAVE_CC_WITH_LIBUNWIND
assert (local.ptr_personality != NULL);
+#else
+ assert (local.ptr__Unwind_GetLanguageSpecificData != NULL);
+ assert (local.ptr__Unwind_SetGR != NULL);
+ assert (local.ptr__Unwind_GetIP != NULL);
+ assert (local.ptr__Unwind_SetIP != NULL);
+ assert (local.ptr__Unwind_GetRegionStart != NULL);
+#endif
PTR_MANGLE (local.ptr__Unwind_Backtrace);
PTR_MANGLE (local.ptr__Unwind_ForcedUnwind);
@@ -96,7 +121,14 @@ __libc_unwind_link_get (void)
#if UNWIND_LINK_FRAME_STATE_FOR
PTR_MANGLE (local.ptr___frame_state_for);
#endif
+#ifndef HAVE_CC_WITH_LIBUNWIND
PTR_MANGLE (local.ptr_personality);
+#else
+ PTR_MANGLE (local.ptr__Unwind_GetLanguageSpecificData);
+ PTR_MANGLE (local.ptr__Unwind_SetGR);
+ PTR_MANGLE (local.ptr__Unwind_SetIP);
+ PTR_MANGLE (local.ptr__Unwind_GetRegionStart);
+#endif
__libc_lock_lock (lock);
if (atomic_load_relaxed (&global_libgcc_handle) != NULL)
diff --git a/nptl/pthread_cancel.c b/nptl/pthread_cancel.c
index 5fe31c266c..6757ea2ba4 100644
--- a/nptl/pthread_cancel.c
+++ b/nptl/pthread_cancel.c
@@ -87,7 +87,7 @@ __pthread_cancel (pthread_t th)
{
struct unwind_link *unwind_link = __libc_unwind_link_get ();
if (unwind_link == NULL)
- __libc_fatal (LIBGCC_S_SO
+ __libc_fatal (UNWIND_SONAME
" must be installed for pthread_cancel to work\n");
}
#endif
diff --git a/nptl/pthread_exit.c b/nptl/pthread_exit.c
index 4c15171992..1ed5ed52c4 100644
--- a/nptl/pthread_exit.c
+++ b/nptl/pthread_exit.c
@@ -28,7 +28,7 @@ __pthread_exit (void *value)
{
struct unwind_link *unwind_link = __libc_unwind_link_get ();
if (unwind_link == NULL)
- __libc_fatal (LIBGCC_S_SO
+ __libc_fatal (UNWIND_SONAME
" must be installed for pthread_exit to work\n");
}
#endif
diff --git a/shlib-versions b/shlib-versions
index b1fd0a5eab..32093228c4 100644
--- a/shlib-versions
+++ b/shlib-versions
@@ -68,3 +68,5 @@ libmvec=1
# The malloc debug library
libc_malloc_debug=0
+
+libunwind=1
diff --git a/sysdeps/generic/unwind-link.h b/sysdeps/generic/unwind-link.h
index 1150cda219..ea39e525d2 100644
--- a/sysdeps/generic/unwind-link.h
+++ b/sysdeps/generic/unwind-link.h
@@ -34,6 +34,12 @@ unwind_arch_adjustment (void *prev, void *addr)
# include <pointer_guard.h>
# include <unwind-resume.h>
+# ifndef HAVE_CC_WITH_LIBUNWIND
+# define UNWIND_SONAME LIBGCC_S_SO
+# else
+# define UNWIND_SONAME LIBUNWIND_SO
+# endif
+
# if UNWIND_LINK_FRAME_STATE_FOR
struct frame_state;
# endif
@@ -50,7 +56,14 @@ struct unwind_link
#if UNWIND_LINK_FRAME_STATE_FOR
struct frame_state *(*ptr___frame_state_for) (void *, struct frame_state *);
#endif
+#ifndef HAVE_CC_WITH_LIBUNWIND
_Unwind_Reason_Code (*ptr_personality) PERSONALITY_PROTO;
+#else
+ __typeof (_Unwind_GetLanguageSpecificData) *ptr__Unwind_GetLanguageSpecificData;
+ __typeof (_Unwind_SetGR) *ptr__Unwind_SetGR;
+ __typeof (_Unwind_SetIP) *ptr__Unwind_SetIP;
+ __typeof (_Unwind_GetRegionStart) *ptr__Unwind_GetRegionStart;
+#endif
UNWIND_LINK_EXTRA_FIELDS
};
diff --git a/sysdeps/generic/unwind-resume.c b/sysdeps/generic/unwind-resume.c
index de176a1f72..18c40501bb 100644
--- a/sysdeps/generic/unwind-resume.c
+++ b/sysdeps/generic/unwind-resume.c
@@ -38,11 +38,43 @@ _Unwind_Resume (struct _Unwind_Exception *exc)
}
#endif
+#ifndef HAVE_CC_WITH_LIBUNWIND
_Unwind_Reason_Code
__gcc_personality_v0 PERSONALITY_PROTO
{
return UNWIND_LINK_PTR (link (), personality) PERSONALITY_ARGS;
}
+#else
+void *
+_Unwind_GetLanguageSpecificData (struct _Unwind_Context *context)
+{
+ return UNWIND_LINK_PTR (link (), _Unwind_GetLanguageSpecificData) (context);
+}
+
+void
+_Unwind_SetGR (struct _Unwind_Context *context, int index, _Unwind_Word val)
+{
+ UNWIND_LINK_PTR (link (), _Unwind_SetGR) (context, index, val);
+}
+
+_Unwind_Ptr
+_Unwind_GetIP (struct _Unwind_Context *context)
+{
+ return UNWIND_LINK_PTR (link (), _Unwind_GetIP) (context);
+}
+
+void
+_Unwind_SetIP (struct _Unwind_Context *context, _Unwind_Ptr val)
+{
+ UNWIND_LINK_PTR (link (), _Unwind_SetIP) (context, val);
+}
+
+_Unwind_Ptr
+_Unwind_GetRegionStart (struct _Unwind_Context *context)
+{
+ return UNWIND_LINK_PTR (link (), _Unwind_GetRegionStart) (context);
+}
+#endif
_Unwind_Reason_Code
_Unwind_ForcedUnwind (struct _Unwind_Exception *exc, _Unwind_Stop_Fn stop,
--
2.43.0
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