[PATCH] weak symbols need to be "defined" weak but "declared" strong

Vineet Gupta Vineet.Gupta1 at synopsys.com
Wed Apr 16 10:36:01 UTC 2014


Patch "LT.old: Make __errno_location/__h_errno_location thread safe"
uncovered yet another bug with static linking and errno (hopefully this
is last of them all).

Currently, __errno_location is declared weak but is defined strong.
While this provides with the desired weak semantics in dso, it
is subtly broken in static links.

Quoting Joern Rennecke (ARC gcc expert):

| I think the issue is that you declare the function as weak in the
| header file.  That is a rare instance where you want the reference
| use declaration that differs a bit from the definition.
| If the reference uses a weakly declared function, that creates a
| weakref, i.e. the linker won't bother to look for this symbol at
|  all - if it gets linked in for some other reason, fine,
| otherwise, it stays zero.

So the solution to declare strong, define weak.

Supporting data
-----------------
orig code: ARM mmap wrapper (LT.old build + my prev patch for errno)

_mmap:
    @ args = 8, pretend = 0, frame = 0
    @ frame_needed = 0, uses_anonymous_args = 0
    stmfd    sp!, {r4, r5, r7, lr}
    ldr    r5, [sp, #20]
    movs    ip, r5, asl #20
    beq    .L2
    bl    __errno_location(PLT)
    mov    r3, #22
    str    r3, [r0, #0]
    mvn    r0, #0
...
...
   .weak	__errno_location

A statically linked hello world program which uses mmap too.
As we can see__errno_location is completely gone - which is
semantically wrong - we need functional errno.

00008274 <__GI_mmap>:
    8274:    e92d40b0     push    {r4, r5, r7, lr}
    8278:    e59d5014     ldr    r5, [sp, #20]
    827c:    e1b0ca05     lsls    ip, r5, #20
    8280:    0a000004     beq    8298
    8284:    e320f000     nop    {0}
		          ^^^^^^^^^^
    8288:    e3a03016     mov    r3, #22
    828c:    e5803000     str    r3, [r0]
    8290:    e3e00000     mvn    r0, #0

This in turn is due to a fixup in ARM ld which transforms branch-to-null
into a nop. It is better than crashing but still wrong since errno
handling is removed.

With the patch, errno_location is restored back in test program.

00008274 <__GI_mmap>:
    8274:	e92d40b0 	push	{r4, r5, r7, lr}
    8278:	e59d5014 	ldr	r5, [sp, #20]
    827c:	e1b0ca05 	lsls	ip, r5, #20
    8280:	0a000004 	beq	8298 <__GI_mmap+0x24>
    8284:	eb000010 	bl	82cc <__errno_location>
    8288:	e3a03016 	mov	r3, #22
    828c:	e5803000 	str	r3, [r0]

Cc: Christian Ruppert <christian.ruppert at abilis.com>
CC: Francois Bedard <Francois.Bedard at synopsys.com>
Cc: Anton Kolesov <Anton.Kolesov at synopsys.com>
Cc: Joern Rennecke  <joern.rennecke at embecosm.com>
Cc: Jeremy Bennett <jeremy.bennett at embecosm.com>
Cc: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>
Cc: Peter Korsgaard <peter at korsgaard.com>
Cc: Khem Raj <raj.khem at gmail
Signed-off-by: Vineet Gupta <vgupta at synopsys.com>
---
 include/netdb.h                          | 3 ---
 libc/misc/internals/__errno_location.c   | 2 +-
 libc/misc/internals/__h_errno_location.c | 2 +-
 libc/sysdeps/linux/common/bits/errno.h   | 3 ---
 4 files changed, 2 insertions(+), 8 deletions(-)

diff --git a/include/netdb.h b/include/netdb.h
index 0f361bb6f662..7ce01c24d8b4 100644
--- a/include/netdb.h
+++ b/include/netdb.h
@@ -58,9 +58,6 @@ __BEGIN_DECLS
 
 /* Function to get address of global `h_errno' variable.  */
 extern int *__h_errno_location (void) __THROW __attribute__ ((__const__));
-#ifdef _LIBC
-extern int weak_const_function *__h_errno_location(void);
-#endif
 
 /* Macros for accessing h_errno from inside libc.  */
 #ifdef _LIBC
diff --git a/libc/misc/internals/__errno_location.c b/libc/misc/internals/__errno_location.c
index 9bbc2d779653..6c359f933d16 100644
--- a/libc/misc/internals/__errno_location.c
+++ b/libc/misc/internals/__errno_location.c
@@ -12,7 +12,7 @@
 extern int errno;
 #endif
 
-int *__errno_location(void)
+int weak_const_function *__errno_location(void)
 {
     return &errno;
 }
diff --git a/libc/misc/internals/__h_errno_location.c b/libc/misc/internals/__h_errno_location.c
index b30859e81f19..c510c8143eae 100644
--- a/libc/misc/internals/__h_errno_location.c
+++ b/libc/misc/internals/__h_errno_location.c
@@ -12,7 +12,7 @@
 extern int h_errno;
 #endif
 
-int *__h_errno_location(void)
+int weak_const_function *__h_errno_location(void)
 {
     return &h_errno;
 }
diff --git a/libc/sysdeps/linux/common/bits/errno.h b/libc/sysdeps/linux/common/bits/errno.h
index 611b8359001a..7c0aeb179a75 100644
--- a/libc/sysdeps/linux/common/bits/errno.h
+++ b/libc/sysdeps/linux/common/bits/errno.h
@@ -42,9 +42,6 @@
 # ifndef __ASSEMBLER__
 /* Function to get address of global `errno' variable.  */
 extern int *__errno_location (void) __THROW __attribute__ ((__const__));
-#  ifdef _LIBC
-extern int weak_const_function *__errno_location(void);
-#  endif
 
 #  ifdef __UCLIBC_HAS_THREADS__
 /* When using threads, errno is a per-thread value.  */
-- 
1.8.3.2



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