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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Sven Schnelle <svens@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.4 14/20] s390/entry: save the caller of psw_idle
Date:   Mon, 26 Apr 2021 09:30:05 +0200
Message-Id: <20210426072817.152717905@linuxfoundation.org>
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From: Vasily Gorbik <gor@linux.ibm.com>

[ Upstream commit a994eddb947ea9ebb7b14d9a1267001699f0a136 ]

Currently psw_idle does not allocate a stack frame and does not
save its r14 and r15 into the save area. Even though this is valid from
call ABI point of view, because psw_idle does not make any calls
explicitly, in reality psw_idle is an entry point for controlled
transition into serving interrupts. So, in practice, psw_idle stack
frame is analyzed during stack unwinding. Depending on build options
that r14 slot in the save area of psw_idle might either contain a value
saved by previous sibling call or complete garbage.

  [task    0000038000003c28] do_ext_irq+0xd6/0x160
  [task    0000038000003c78] ext_int_handler+0xba/0xe8
  [task   *0000038000003dd8] psw_idle_exit+0x0/0x8 <-- pt_regs
 ([task    0000038000003dd8] 0x0)
  [task    0000038000003e10] default_idle_call+0x42/0x148
  [task    0000038000003e30] do_idle+0xce/0x160
  [task    0000038000003e70] cpu_startup_entry+0x36/0x40
  [task    0000038000003ea0] arch_call_rest_init+0x76/0x80

So, to make a stacktrace nicer and actually point for the real caller of
psw_idle in this frequently occurring case, make psw_idle save its r14.

  [task    0000038000003c28] do_ext_irq+0xd6/0x160
  [task    0000038000003c78] ext_int_handler+0xba/0xe8
  [task   *0000038000003dd8] psw_idle_exit+0x0/0x6 <-- pt_regs
 ([task    0000038000003dd8] arch_cpu_idle+0x3c/0xd0)
  [task    0000038000003e10] default_idle_call+0x42/0x148
  [task    0000038000003e30] do_idle+0xce/0x160
  [task    0000038000003e70] cpu_startup_entry+0x36/0x40
  [task    0000038000003ea0] arch_call_rest_init+0x76/0x80

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/s390/kernel/entry.S | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
index c544b7a11ebb..5cba1815b8f8 100644
--- a/arch/s390/kernel/entry.S
+++ b/arch/s390/kernel/entry.S
@@ -993,6 +993,7 @@ ENDPROC(ext_int_handler)
  * Load idle PSW. The second "half" of this function is in .Lcleanup_idle.
  */
 ENTRY(psw_idle)
+	stg	%r14,(__SF_GPRS+8*8)(%r15)
 	stg	%r3,__SF_EMPTY(%r15)
 	larl	%r1,.Lpsw_idle_lpsw+4
 	stg	%r1,__SF_EMPTY+8(%r15)
-- 
2.30.2