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Date:   Sat, 29 Aug 2020 22:23:30 -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 04:37:32PM +0200, Hauke Mehrtens wrote:
>On 8/29/20 3:56 PM, Sasha Levin wrote:
>> 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.
>>
>
>I successfully compiled a kernel 5.4.61 with this patch on top with GCC
>8.4 for MIPS 64 big and little Endian.
>
>What was broken in your compile test?

See https://lore.kernel.org/stable/bfdce3ef-5fe9-8dab-1695-be3d33727529@roeck-us.net/

-- 
Thanks,
Sasha

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