weak symbols need to be "defined" weak but "declared" strong
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@abilis.com>
CC: Francois Bedard <Francois.Bedard@synopsys.com>
Cc: Anton Kolesov <Anton.Kolesov@synopsys.com>
Cc: Joern Rennecke <joern.rennecke@embecosm.com>
Cc: Jeremy Bennett <jeremy.bennett@embecosm.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
Vineet Gupta