Re: [PATCH AUTOSEL 5.4 10/58] mips: vdso: fix 'jalr t9' crash in vdso code

[Sasha Levin]

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@xxxxxxxxx>
> >
> > [ 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@xxxxxxxxx>
> > Signed-off-by: Victor Kamensky <kamensky@xxxxxxxxx>
> > Signed-off-by: Paul Burton <paulburton@xxxxxxxxxx>
> > Cc: linux-mips@xxxxxxxxxxxxxxx
> > Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
> > Cc: James Hogan <jhogan@xxxxxxxxxx>
> > Cc: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> > Cc: richard.purdie@xxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> > ---
> >  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