linux-kernel - Re: [PATCH v2] MIPS: Avoid VDSO ABI breakage due to global register variable

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Message-ID: <fce2be75-9a68-37eb-723a-99d010e77132@arm.com>
Date:   Fri, 3 Jan 2020 09:15:10 +0000
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Paul Burton <paulburton@...nel.org>
Cc:     linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <christian.brauner@...onical.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] MIPS: Avoid VDSO ABI breakage due to global register
 variable

Hi Paul,

On 1/3/20 12:42 AM, Paul Burton wrote:
> Using -ffixed-gp wouldn't be correct for the VDSO - the VDSO itself is
> position independent code, and will need to use $gp to access the GOT
> which is part of how position-independence is achieved (technically you
> could access the GOT using another register of course but you'd need
> some way to persuade the compiler to break with convention & you'd gain
> nothing meaningful since you'd need to use some other register anyway).
> If we use -ffixed-gp then we're telling GCC not to use $gp, and that
> doesn't make sense. If we consider -ffixed-gp as telling GCC not to use
> $gp as a general purpose register then it's meaningless because $gp
> already has a specific use & isn't used as a general purpose register.
> If we consider -ffixed-gp as telling GCC not to use $gp at all then it
> doesn't make sense because it needs to in order to access the GOT.
> 
> In terms of GCC's flags we'd want to use -fcall-saved-gp, but that would
> just be telling GCC information it already knows about the n32 & n64
> ABIs & indeed it seems to have no effect at all on the way GCC handles
> the global register variable - it doesn't cause gcc to save & restore
> $gp with the global register variable present, so you gain nothing.
> 
> We could use -ffixed-gp for the kernel proper (& not the VDSO), but:
> 
> 1) The kernel builds as non-PIC code with no $gp-based optimizations
>    enabled, and since this has been fine forever it seems safe to expect
>    the compiler not to start using $gp in new ways.
> 
> 2) It would be a separate issue to fixing the VDSO anyway.

Makes totally sense. Thanks for the explanation.

-- 
Regards,
Vincenzo