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

[Julian Brown <julian@codesourcery.com>]

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?

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.

Julian