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Re: [PATCH v3] MIPS: kernel: elf: Improve the overall ABI and FPU mode c

To: Måns Rullgård <mans@mansr.com>
Subject: Re: [PATCH v3] MIPS: kernel: elf: Improve the overall ABI and FPU mode checks
From: Markos Chandras <markos.chandras@imgtec.com>
Date: Tue, 24 Feb 2015 13:52:25 +0000
Cc: <linux-mips@linux-mips.org>, Matthew Fortune <Matthew.Fortune@imgtec.com>, Paul Burton <paul.burton@imgtec.com>
In-reply-to: <yw1xwq3778k2.fsf@unicorn.mansr.com>
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Hi,

On Tue, Feb 24, 2015 at 01:17:33PM +0000, Måns Rullgård wrote:
> This patch (well, the variant that made it into 4.0-rc1) breaks
> MIPS_ABI_FP_DOUBLE (the gcc default) apps on MIPS32.
> 

Thanks for the report.

> > +void mips_set_personality_fp(struct arch_elf_state *state)
> > +{
> > +   /*
> > +    * This function is only ever called for O32 ELFs so we should
> > +    * not be worried about N32/N64 binaries.
> > +    */
> >
> > -   case MIPS_ABI_FP_XX:
> > -   case MIPS_ABI_FP_ANY:
> > -           if (!config_enabled(CONFIG_MIPS_O32_FP64_SUPPORT))
> > -                   set_thread_flag(TIF_32BIT_FPREGS);
> > -           else
> > -                   clear_thread_flag(TIF_32BIT_FPREGS);
> > +   if (!config_enabled(CONFIG_MIPS_O32_FP64_SUPPORT))
> > +           return;
> 
> The problem is here.  In a 32-bit configuration, MIPS_O32_FP64_SUPPORT
> is always disabled, so the FP mode doesn't get set.  Simply deleting
> those two lines makes things work again, but that's probably not the
> right fix.
> 
I had the impression that the loader would have set the FP mode earlier on.
But that only may happen with the latest version of the tools.

Perhaps instead of dropping these two lines we need a similar check on the
arch_elf_pt_proc so we don't mess with the default FPI abi?

Having said that, dropping these two lines should be fine, it just means you
do a little bit of extra work when loading your ELF files to check for ABI
compatibility which shouldn't matter in your case.

Matthew what do you think?

-- 
markos

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