From: Jakub Jelinek <jakub@redhat.com>
Date: Thu, 3 Dec 2020 12:33:44 +0000 (+0100)
Subject: x86: Fix THREAD_SELF definition to avoid ld.so crash (bug 27004)
X-Git-Tag: glibc-2.33~244
X-Git-Url: https://sourceware.org/git/?a=commitdiff_plain;h=1d9cbb96082e646de7515a1667efa041ffb79958;p=glibc.git

x86: Fix THREAD_SELF definition to avoid ld.so crash (bug 27004)

The previous definition of THREAD_SELF did not tell the compiler
that %fs (or %gs) usage is invalid for the !DL_LOOKUP_GSCOPE_LOCK
case in _dl_lookup_symbol_x.  As a result, ld.so could try to use the
TCB before it was initialized.

As the comment in tls.h explains, asm volatile is undesirable here.
Using the __seg_fs (or __seg_gs) namespace does not interfere with
optimization, and expresses that THREAD_SELF is potentially trapping.
---

diff --git a/sysdeps/i386/nptl/tls.h b/sysdeps/i386/nptl/tls.h
index b74347bacd..ab98c24a02 100644
--- a/sysdeps/i386/nptl/tls.h
+++ b/sysdeps/i386/nptl/tls.h
@@ -234,11 +234,16 @@ tls_fill_user_desc (union user_desc_init *desc,
    assignments like
 	pthread_descr self = thread_self();
    do not get optimized away.  */
-# define THREAD_SELF \
+# if __GNUC_PREREQ (6, 0)
+#  define THREAD_SELF \
+  (*(struct pthread *__seg_gs *) offsetof (struct pthread, header.self))
+# else
+#  define THREAD_SELF \
   ({ struct pthread *__self;						      \
      asm ("movl %%gs:%c1,%0" : "=r" (__self)				      \
 	  : "i" (offsetof (struct pthread, header.self)));		      \
      __self;})
+# endif
 
 /* Magic for libthread_db to know how to do THREAD_SELF.  */
 # define DB_THREAD_SELF \
diff --git a/sysdeps/x86_64/nptl/tls.h b/sysdeps/x86_64/nptl/tls.h
index a08bf972de..ccb5f24d92 100644
--- a/sysdeps/x86_64/nptl/tls.h
+++ b/sysdeps/x86_64/nptl/tls.h
@@ -180,11 +180,16 @@ _Static_assert (offsetof (tcbhead_t, __glibc_unused2) == 0x80,
    assignments like
 	pthread_descr self = thread_self();
    do not get optimized away.  */
-# define THREAD_SELF \
+# if __GNUC_PREREQ (6, 0)
+#  define THREAD_SELF \
+  (*(struct pthread *__seg_fs *) offsetof (struct pthread, header.self))
+# else
+#  define THREAD_SELF \
   ({ struct pthread *__self;						      \
      asm ("mov %%fs:%c1,%0" : "=r" (__self)				      \
 	  : "i" (offsetof (struct pthread, header.self)));	 	      \
      __self;})
+# endif
 
 /* Magic for libthread_db to know how to do THREAD_SELF.  */
 # define DB_THREAD_SELF_INCLUDE  <sys/reg.h> /* For the FS constant.  */