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

[Raoni Fassina Firmino <raoni@linux.ibm.com>]

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< ----

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  Now this semantics are no longer
valid.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@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.  */
 	.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
diff --git a/arch/powerpc/kernel/vdso64/vdso64.lds.S b/arch/powerpc/kernel/vdso64/vdso64.lds.S
index 6164d1a1ba11..2f3c359cacd3 100644
--- a/arch/powerpc/kernel/vdso64/vdso64.lds.S
+++ b/arch/powerpc/kernel/vdso64/vdso64.lds.S
@@ -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;

base-commit: 76c057c84d286140c6c416c3b4ba832cd1d8984e
-- 
2.26.2