[PATCH] Fix thumb calls via PLT on ARM/SymbianOS

Richard Earnshaw rearnsha@gcc.gnu.org
Wed Mar 16 16:29:00 GMT 2005


On Wed, 2005-03-16 at 14:49, Julian Brown wrote:
> Daniel Jacobowitz wrote:
> > On Wed, Mar 16, 2005 at 02:23:12PM +0000, Richard Earnshaw wrote:
> > 
> >>On Wed, 2005-03-16 at 14:17, Julian Brown wrote:
> >>
> >>>Hi,
> >>>
> >>>This patch fixes calls made from thumb mode via the PLT on SymbianOS. 
> >>>PLT entries are written in ARM mode, but previously they were being 
> >>>called in thumb mode, with their address incorrectly offset by -4 
> >>>(PLT_THUMB_STUB_SIZE).
> >>>
> >>>Now, the target address has been corrected and the thumb BL instruction 
> >>>is rewritten as BLX to perform the mode switch before attempting to 
> >>>execute the PLT entry. (BLX is an armv5t instruction, but as SymbianOS 
> >>>is only targeted at armv5t+, this should always be OK.)
> >>>
> >>>The hardwired "4" for the thumb stub size on non-SymbianOS targets has 
> >>>also been rewritten as PLT_THUMB_STUB_SIZE.
> >>>
> >>>OK to apply?
> >>
> >>This sounds like a hack.  I'd much rather a generic solution were found
> >>(and which permitted the blx optimization if available).
> > 
> > 
> > The SymbianOS bits sound generally right to me.  The fact that the -4
> > bias is currently included is just a bug; that's from the code to
> > generate a Thumb header on PLT entries, which is already disabled
> > for SymbianOS.
> > 
> > OTOH, it may be missing some error checks - what if it's _not_ a BL? 
> > Could we reach here?
> 
> I assumed not: this is an R_ARM_THM_PC22, alias R_ARM_THM_CALL 
> relocation. Can that sensibly point at anything else?
> 

Ah!  Yep, that can only apply to a thumb BL instruction.

> Would it be better to add a --target-arch (or something) flag to 
> binutils at this point? That would tidy up a previous patch of mine, 
> too. I don't know if any other targets have, or need, such an option.

Yep, I think that would be a good idea.  Ideally we'd want a linker
command-line option as well, but a configuration option would be a good
first step.

R.



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