FAILED: patch "[PATCH] powerpc/64/signal: Fix regression in __kernel_sigtramp_rt64()" failed to apply to 5.10-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 24321ac668e452a4942598533d267805f291fdc9 Mon Sep 17 00:00:00 2001
From: Raoni Fassina Firmino <raoni@linux.ibm.com>
Date: Mon, 1 Feb 2021 17:05:05 -0300
Subject: [PATCH] powerpc/64/signal: Fix regression in __kernel_sigtramp_rt64()
 semantics

Commit 0138ba5783ae ("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 Now this semantics are no longer valid.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 0138ba5783ae 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: 0138ba5783ae ("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

diff --git a/arch/powerpc/kernel/vdso64/sigtramp.S b/arch/powerpc/kernel/vdso64/sigtramp.S
index bbf68cd01088..2d4067561293 100644
--- a/arch/powerpc/kernel/vdso64/sigtramp.S
+++ b/arch/powerpc/kernel/vdso64/sigtramp.S
@@ -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
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;