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Re: [PATCH] thumb fixes
On Fri, Sep 05, 2008 at 10:39:09AM +0100, Richard Earnshaw wrote:
> Hmm, I remember now. I think this is an LD bug. When thumb code calls
> a PLT entry it needs to use an interworking call sequence (the BL needs
> to be turned into BLX on v5t). LD doesn't realize this because the PLT
> sequence is internally generated.
> I believe this has been fixed in FSF binutils, but probably not in
> NetBSD's in-tree version (and I can't remember if this was ever fixed in
> a GPLv2 release).
Ok, I was already browsing through
and others which are involved in generating the plt stuff.
> > If there only was a way to force the plt section to be in ARM mode...
> The PLT *is* in ARM mode... Your problem is that you're calling it from
> Thumb :-(
> There is a work-around, but it costs a bit in both performance and code
> size. You can build all your thumb code with -mlong-calls. Doing this
> will mean that the compiler will only use BL when it can prove that the
> call will be non-interworking at compile time (ie a static function).
-mlong-calls indeed fixes /bin/cat, when ld.elf_so is in ARM mode.
/bin/sh instead segfaults, but that problem seems to be within libc
which propably has a number of problematic entry and exit points.
I'm running /bin/sh with libraries and linkers in a chroot, so gdb
can not get me a backtrace easily.
With -mlong-calls the plt sections looks ok:
8908: e52de004 str lr, [sp, #-4]!
890c: e59fe004 ldr lr, [pc, #4] ; 8918 <.plt+0x10>
8910: e08fe00e add lr, pc, lr
8914: e5bef008 ldr pc, [lr, #8]!
8918: 000091f0 streqd r9, [r0], -r0
891c: e28fc600 add ip, pc, #0 ; 0x0
8920: e28cca09 add ip, ip, #36864 ; 0x9000
8924: e5bcf1f0 ldr pc, [ip, #496]!
8928: e28fc600 add ip, pc, #0 ; 0x0
892c: e28cca09 add ip, ip, #36864 ; 0x9000
8930: e5bcf1e8 ldr pc, [ip, #488]!
8934: e28fc600 add ip, pc, #0 ; 0x0
8938: e28cca09 add ip, ip, #36864 ; 0x9000
893c: e5bcf1e0 ldr pc, [ip, #480]!
8940: e28fc600 add ip, pc, #0 ; 0x0
8944: e28cca09 add ip, ip, #36864 ; 0x9000
8948: e5bcf1d8 ldr pc, [ip, #472]!
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