[PATCH] powerpc/64/signal: Fix regression in __kernel_sigtramp_rt64 semantics

[Nicholas Piggin]

Excerpts from Raoni Fassina Firmino's message of February 2, 2021 6:05 am:
> Tested on powerpc64 and powerpc64le, with a glibc build and running the
> affected glibc's testcase[2], inspected that glibc's backtrace() now gives
> the correct result and gdb backtrace also keeps working as before.
> 
> I believe this should be backported to releases 5.9 and 5.10 as userspace
> is affected in this releases.
> 
> ---- 8< ----

Thanks for this, I don't know the glibc code but the kernel change seems 
okay to me.

Thanks,
Nick

> 
> A Change[1] in __kernel_sigtramp_rt64 VDSO and trampoline code introduced a
> regression in the way glibc's backtrace()[2] detects the signal-handler
> stack frame.  Apart from the practical implications, __kernel_sigtram_rt64
> was a VDSO with the semantics that it is a function you can call from
> userspace to end a signal handling.  Now this semantics are no longer
> valid.
> 
> I believe the aforementioned change affects all releases since 5.9.
> 
> This patch tries to fix both the semantics and practical aspect of
> __kernel_sigtramp_rt64 returning it to the previous code, whilst keeping
> the intended behavior from[1] by adding a new symbol to serve as the jump
> target from the kernel to the trampoline. Now the trampoline has two parts,
> an new entry point and the old return point.
> 
> [1] commit 0138ba5783ae0dcc799ad401a1e8ac8333790df9 ("powerpc/64/signal:
>     Balance return predictor stack in signal trampoline")
> [2] https://lists.ozlabs.org/pipermail/linuxppc-dev/2021-January/223194.html
> 
> Fixes: 0138ba5783ae ("powerpc/64/signal: Balance return predictor stack in signal trampoline")
> Signed-off-by: Raoni Fassina Firmino <raoni at linux.ibm.com>
> ---
>  arch/powerpc/kernel/vdso64/sigtramp.S   | 9 ++++++++-
>  arch/powerpc/kernel/vdso64/vdso64.lds.S | 2 +-
>  2 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/vdso64/sigtramp.S b/arch/powerpc/kernel/vdso64/sigtramp.S
> index bbf68cd01088..f0fd8d2a9fc4 100644
> --- a/arch/powerpc/kernel/vdso64/sigtramp.S
> +++ b/arch/powerpc/kernel/vdso64/sigtramp.S
> @@ -15,11 +15,18 @@
>  
>  	.text
>  
> +/* __kernel_start_sigtramp_rt64 and __kernel_sigtramp_rt64 together
> +   are one function split in two parts. The kernel jumps to the former
> +   and the signal handler indirectly (by blr) returns to the latter.
> +   __kernel_sigtramp_rt64 needs to point to the return address so
> +   glibc can correctly identify the trampoline stack frame.  */

Are you planning to update glibc to cope with this as well? Any idea 
about musl? If so, including version numbers would be good (not that
it's really a problem to carry this patch around).

I was just about to ask to turn the comment into kernel style, but the
whole file has this style so nevermind about that! :)

Thanks,
Nick