[Qemu-devel] [PATCH v2] linux-user: ppc64: don't use volatile register during safe_syscall

[Qemu-devel] [PATCH v2] linux-user: ppc64: don't use volatile register during safe_syscall

[Shivaprasad G Bhat]

r11 is a volatile register on PPC as per calling conventions.
The safe_syscall code uses it to check if the signal_pending
is set during the safe_syscall. When a syscall is interrupted
on return from signal handling, the r11 might be corrupted
before we retry the syscall leading to a crash. The registers
r0-r13 are not to be used here as they have
volatile/designated/reserved usages. Change the code to use
r14 which is non-volatile and is appropriate for local use in
safe_syscall.

Steps to reproduce:
On PPC host, issue `qemu-x86_64 /usr/bin/cc -E -`
Attempt Ctrl-C, the issue is reproduced.

Reference:
https://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html#REG

Signed-off-by: Shivaprasad G Bhat <sbhat@linux.vnet.ibm.com>
Tested-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
---
v1: https://lists.gnu.org/archive/html/qemu-devel/2018-07/msg05089.html
Changes from v1:
   Fixed the commit message as suggested

 linux-user/host/ppc64/safe-syscall.inc.S |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/linux-user/host/ppc64/safe-syscall.inc.S b/linux-user/host/ppc64/safe-syscall.inc.S
index d30050a67c..b0cbbe6a69 100644
--- a/linux-user/host/ppc64/safe-syscall.inc.S
+++ b/linux-user/host/ppc64/safe-syscall.inc.S
@@ -49,7 +49,7 @@ safe_syscall_base:
 	 *               and returns the result in r3
 	 * Shuffle everything around appropriately.
 	 */
-	mr	11, 3	/* signal_pending */
+	mr	14, 3	/* signal_pending */
 	mr	0, 4	/* syscall number */
 	mr	3, 5	/* syscall arguments */
 	mr	4, 6
@@ -67,7 +67,7 @@ safe_syscall_base:
 	 */
 safe_syscall_start:
 	/* if signal_pending is non-zero, don't do the call */
-	lwz	12, 0(11)
+	lwz	12, 0(14)
 	cmpwi	0, 12, 0
 	bne-	0f
 	sc

Re: [Qemu-devel] [Qemu-ppc] [PATCH v2] linux-user: ppc64: don't use volatile register during safe_syscall

[David Gibson]

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On Thu, Jul 26, 2018 at 01:56:19PM +0530, Shivaprasad G Bhat wrote:
> r11 is a volatile register on PPC as per calling conventions.
> The safe_syscall code uses it to check if the signal_pending
> is set during the safe_syscall. When a syscall is interrupted
> on return from signal handling, the r11 might be corrupted
> before we retry the syscall leading to a crash. The registers
> r0-r13 are not to be used here as they have
> volatile/designated/reserved usages. Change the code to use
> r14 which is non-volatile and is appropriate for local use in
> safe_syscall.
> 
> Steps to reproduce:
> On PPC host, issue `qemu-x86_64 /usr/bin/cc -E -`
> Attempt Ctrl-C, the issue is reproduced.
> 
> Reference:
> https://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html#REG
> 
> Signed-off-by: Shivaprasad G Bhat <sbhat@linux.vnet.ibm.com>
> Tested-by: Laurent Vivier <laurent@vivier.eu>
> Reviewed-by: Laurent Vivier <laurent@vivier.eu>

Applied to ppc-for-3.0.

> ---
> v1: https://lists.gnu.org/archive/html/qemu-devel/2018-07/msg05089.html
> Changes from v1:
>    Fixed the commit message as suggested
> 
>  linux-user/host/ppc64/safe-syscall.inc.S |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/linux-user/host/ppc64/safe-syscall.inc.S b/linux-user/host/ppc64/safe-syscall.inc.S
> index d30050a67c..b0cbbe6a69 100644
> --- a/linux-user/host/ppc64/safe-syscall.inc.S
> +++ b/linux-user/host/ppc64/safe-syscall.inc.S
> @@ -49,7 +49,7 @@ safe_syscall_base:
>  	 *               and returns the result in r3
>  	 * Shuffle everything around appropriately.
>  	 */
> -	mr	11, 3	/* signal_pending */
> +	mr	14, 3	/* signal_pending */
>  	mr	0, 4	/* syscall number */
>  	mr	3, 5	/* syscall arguments */
>  	mr	4, 6
> @@ -67,7 +67,7 @@ safe_syscall_base:
>  	 */
>  safe_syscall_start:
>  	/* if signal_pending is non-zero, don't do the call */
> -	lwz	12, 0(11)
> +	lwz	12, 0(14)
>  	cmpwi	0, 12, 0
>  	bne-	0f
>  	sc
> 
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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Re: [Qemu-devel] [Qemu-ppc] [PATCH v2] linux-user: ppc64: don't use volatile register during safe_syscall

[Richard Henderson]

On 07/26/2018 08:13 PM, David Gibson wrote:
> On Thu, Jul 26, 2018 at 01:56:19PM +0530, Shivaprasad G Bhat wrote:
>> r11 is a volatile register on PPC as per calling conventions.
>> The safe_syscall code uses it to check if the signal_pending
>> is set during the safe_syscall. When a syscall is interrupted
>> on return from signal handling, the r11 might be corrupted
>> before we retry the syscall leading to a crash. The registers
>> r0-r13 are not to be used here as they have
>> volatile/designated/reserved usages. Change the code to use
>> r14 which is non-volatile and is appropriate for local use in
>> safe_syscall.
>>
>> Steps to reproduce:
>> On PPC host, issue `qemu-x86_64 /usr/bin/cc -E -`
>> Attempt Ctrl-C, the issue is reproduced.
>>
>> Reference:
>> https://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html#REG
>>
>> Signed-off-by: Shivaprasad G Bhat <sbhat@linux.vnet.ibm.com>
>> Tested-by: Laurent Vivier <laurent@vivier.eu>
>> Reviewed-by: Laurent Vivier <laurent@vivier.eu>
> 
> Applied to ppc-for-3.0.

No please!  r14 is a call-saved register and needs saving before clobbering.


r~

Re: [Qemu-devel] [Qemu-ppc] [PATCH v2] linux-user: ppc64: don't use volatile register during safe_syscall

[David Gibson]

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On Thu, Jul 26, 2018 at 10:20:11PM -0700, Richard Henderson wrote:
> On 07/26/2018 08:13 PM, David Gibson wrote:
> > On Thu, Jul 26, 2018 at 01:56:19PM +0530, Shivaprasad G Bhat wrote:
> >> r11 is a volatile register on PPC as per calling conventions.
> >> The safe_syscall code uses it to check if the signal_pending
> >> is set during the safe_syscall. When a syscall is interrupted
> >> on return from signal handling, the r11 might be corrupted
> >> before we retry the syscall leading to a crash. The registers
> >> r0-r13 are not to be used here as they have
> >> volatile/designated/reserved usages. Change the code to use
> >> r14 which is non-volatile and is appropriate for local use in
> >> safe_syscall.
> >>
> >> Steps to reproduce:
> >> On PPC host, issue `qemu-x86_64 /usr/bin/cc -E -`
> >> Attempt Ctrl-C, the issue is reproduced.
> >>
> >> Reference:
> >> https://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html#REG
> >>
> >> Signed-off-by: Shivaprasad G Bhat <sbhat@linux.vnet.ibm.com>
> >> Tested-by: Laurent Vivier <laurent@vivier.eu>
> >> Reviewed-by: Laurent Vivier <laurent@vivier.eu>
> > 
> > Applied to ppc-for-3.0.
> 
> No please!  r14 is a call-saved register and needs saving before
> clobbering.

Oops, pulled out now.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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