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Date:   Sat, 29 Aug 2020 09:56:56 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     Hauke Mehrtens <hauke@...ke-m.de>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        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 Sat, Aug 29, 2020 at 03:08:01PM +0200, Hauke Mehrtens wrote:
>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?

It failed building on 5.4. If you'd like it included, please send me a
tested backport for 5.4.

-- 
Thanks,
Sasha

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