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Date:   Wed, 27 Feb 2019 11:45:30 +0000 (UTC)
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicolai Stange <nstange@...e.de>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        stable@...r.kernel.org
Subject: [PATCH] powerpc/32: Clear on-stack exception marker upon exception
 return

Clear the on-stack STACK_FRAME_REGS_MARKER on exception exit in order
to avoid confusing stacktrace like the one below.

Call Trace:
[c0e9dca0] [c01c42a0] print_address_description+0x64/0x2bc (unreliable)
[c0e9dcd0] [c01c4684] kasan_report+0xfc/0x180
[c0e9dd10] [c0895130] memchr+0x24/0x74
[c0e9dd30] [c00a9e38] msg_print_text+0x124/0x574
[c0e9dde0] [c00ab710] console_unlock+0x114/0x4f8
[c0e9de40] [c00adc60] vprintk_emit+0x188/0x1c4
--- interrupt: c0e9df00 at 0x400f330
    LR = init_stack+0x1f00/0x2000
[c0e9de80] [c00ae3c4] printk+0xa8/0xcc (unreliable)
[c0e9df20] [c0c27e44] early_irq_init+0x38/0x108
[c0e9df50] [c0c15434] start_kernel+0x310/0x488
[c0e9dff0] [00003484] 0x3484

With this patch the trace becomes:

Call Trace:
[c0e9dca0] [c01c42c0] print_address_description+0x64/0x2bc (unreliable)
[c0e9dcd0] [c01c46a4] kasan_report+0xfc/0x180
[c0e9dd10] [c0895150] memchr+0x24/0x74
[c0e9dd30] [c00a9e58] msg_print_text+0x124/0x574
[c0e9dde0] [c00ab730] console_unlock+0x114/0x4f8
[c0e9de40] [c00adc80] vprintk_emit+0x188/0x1c4
[c0e9de80] [c00ae3e4] printk+0xa8/0xcc
[c0e9df20] [c0c27e44] early_irq_init+0x38/0x108
[c0e9df50] [c0c15434] start_kernel+0x310/0x488
[c0e9dff0] [00003484] 0x3484

Cc: stable@...r.kernel.org
Cc: Nicolai Stange <nstange@...e.de>

Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
---
 arch/powerpc/kernel/entry_32.S | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
index 96dce6a4b61e..b61cfd29c76f 100644
--- a/arch/powerpc/kernel/entry_32.S
+++ b/arch/powerpc/kernel/entry_32.S
@@ -730,6 +730,9 @@ fast_exception_return:
 	mtcr	r10
 	lwz	r10,_LINK(r11)
 	mtlr	r10
+	/* Clear the exception_marker on the stack to avoid confusing stacktrace */
+	li	r10, 0
+	stw	r10, 8(r11)
 	REST_GPR(10, r11)
 #if defined(CONFIG_PPC_8xx) && defined(CONFIG_PERF_EVENTS)
 	mtspr	SPRN_NRI, r0
@@ -961,6 +964,9 @@ END_FTR_SECTION_IFSET(CPU_FTR_NEED_PAIRED_STWCX)
 	mtcrf	0xFF,r10
 	mtlr	r11
 
+	/* Clear the exception_marker on the stack to avoid confusing stacktrace */
+	li	r10, 0
+	stw	r10, 8(r1)
 	/*
 	 * Once we put values in SRR0 and SRR1, we are in a state
 	 * where exceptions are not recoverable, since taking an
@@ -997,6 +1003,9 @@ exc_exit_restart_end:
 	mtlr	r11
 	lwz	r10,_CCR(r1)
 	mtcrf	0xff,r10
+	/* Clear the exception_marker on the stack to avoid confusing stacktrace */
+	li	r10, 0
+	stw	r10, 8(r1)
 	REST_2GPRS(9, r1)
 	.globl exc_exit_restart
 exc_exit_restart:
-- 
2.13.3

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