[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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