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powerpc/64/signal: Fix regression in __kernel_sigtramp_rt64() semantics

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Commit 0138ba5783 ("powerpc/64/signal: Balance return predictor
stack in signal trampoline") changed __kernel_sigtramp_rt64() VDSO and
trampoline code, and introduced a regression in the way glibc's
backtrace()[1] detects the signal-handler stack frame. Apart from the
practical implications, __kernel_sigtramp_rt64() was a VDSO function
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 Now this semantics are no longer valid.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 behaviour of 0138ba5783 by adding a new symbol
to serve as the jump target from the kernel to the trampoline. Now the
trampoline has two parts, a new entry point and the old return point.

[1] https://lists.ozlabs.org/pipermail/linuxppc-dev/2021-January/223194.html

Fixes: 0138ba5783 ("powerpc/64/signal: Balance return predictor stack in signal trampoline")
Cc: stable@vger.kernel.org # v5.9+
Signed-off-by: Raoni Fassina Firmino <raoni@linux.ibm.com>
Acked-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Minor tweaks to change log formatting, add stable tag]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210201200505.iz46ubcizipnkcxe@work-tp
This commit is contained in:
Raoni Fassina Firmino 2021-02-01 17:05:05 -03:00 committed by Michael Ellerman
parent 66f0a9e058
commit 24321ac668
2 changed files with 11 additions and 2 deletions
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11
arch/powerpc/kernel/vdso64/sigtramp.S
View file

@ -15,11 +15,20 @@
.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.
*/
.balign 8
.balign IFETCH_ALIGN_BYTES
V_FUNCTION_BEGIN(__kernel_sigtramp_rt64)
V_FUNCTION_BEGIN(__kernel_start_sigtramp_rt64)
.Lsigrt_start:
bctrl /* call the handler */
V_FUNCTION_END(__kernel_start_sigtramp_rt64)
V_FUNCTION_BEGIN(__kernel_sigtramp_rt64)
addi r1, r1, __SIGNAL_FRAMESIZE
li r0,__NR_rt_sigreturn
sc

2
arch/powerpc/kernel/vdso64/vdso64.lds.S
View file

@ -131,4 +131,4 @@ VERSION
/*
* Make the sigreturn code visible to the kernel.
*/
VDSO_sigtramp_rt64 = __kernel_sigtramp_rt64;
VDSO_sigtramp_rt64 = __kernel_start_sigtramp_rt64;