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Date:   Sat, 29 Aug 2020 15:08:01 +0200
From:   Hauke Mehrtens <hauke@...ke-m.de>
To:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     Victor Kamensky <kamensky@...co.com>,
        Bruce Ashfield <bruce.ashfield@...il.com>,
        Paul Burton <paulburton@...nel.org>,
        linux-mips@...r.kernel.org, Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        richard.purdie@...uxfoundation.org,
        Tony Ambardar <itugrok@...oo.com>
Subject: Re: [PATCH AUTOSEL 5.4 10/58] mips: vdso: fix 'jalr t9' crash in vdso
 code

On 3/5/20 6:13 PM, Sasha Levin wrote:
> From: Victor Kamensky <kamensky@...co.com>
> 
> [ Upstream commit d3f703c4359ff06619b2322b91f69710453e6b6d ]
> 
> Observed that when kernel is built with Yocto mips64-poky-linux-gcc,
> and mips64-poky-linux-gnun32-gcc toolchain, resulting vdso contains
> 'jalr t9' instructions in its code and since in vdso case nobody
> sets GOT table code crashes when instruction reached. On other hand
> observed that when kernel is built mips-poky-linux-gcc toolchain, the
> same 'jalr t9' instruction are replaced with PC relative function
> calls using 'bal' instructions.
> 
> The difference boils down to -mrelax-pic-calls and -mexplicit-relocs
> gcc options that gets different default values depending on gcc
> target triplets and corresponding binutils. -mrelax-pic-calls got
> enabled by default only in mips-poky-linux-gcc case. MIPS binutils
> ld relies on R_MIPS_JALR relocation to convert 'jalr t9' into 'bal'
> and such relocation is generated only if -mrelax-pic-calls option
> is on.
> 
> Please note 'jalr t9' conversion to 'bal' can happen only to static
> functions. These static PIC calls use mips local GOT entries that
> are supposed to be filled with start of DSO value by run-time linker
> (missing in VDSO case) and they do not have dynamic relocations.
> Global mips GOT entries must have dynamic relocations and they should
> be prevented by cmd_vdso_check Makefile rule.
> 
> Solution call out -mrelax-pic-calls and -mexplicit-relocs options
> explicitly while compiling MIPS vdso code. That would get correct
> and consistent between different toolchains behaviour.
> 
> Reported-by: Bruce Ashfield <bruce.ashfield@...il.com>
> Signed-off-by: Victor Kamensky <kamensky@...co.com>
> Signed-off-by: Paul Burton <paulburton@...nel.org>
> Cc: linux-mips@...r.kernel.org
> Cc: Ralf Baechle <ralf@...ux-mips.org>
> Cc: James Hogan <jhogan@...nel.org>
> Cc: Vincenzo Frascino <vincenzo.frascino@....com>
> Cc: richard.purdie@...uxfoundation.org
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>  arch/mips/vdso/Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 

Hi Sasha,

Why was this not added to the 5.4 stable branch?

Some OpenWrt users ran into this problem with kernel 5.4 on MIPS64 [0].
We backported this patch on our own in OpenWrt [1], but it should be
added to the sable branch in my opinion as it fixes a real problem.

@Sasha: Can you add it to the 5.4 stable branch or should I send some
special email?

[0]: https://bugs.openwrt.org/index.php?do=details&task_id=3277
[1]: https://git.openwrt.org/2932b4d05e1d212259c2903fd9cebcee5616680b

Hauke



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