Fix i686 memchr overflow calculation (BZ#21182)

Message ID	1489512487-24860-1-git-send-email-adhemerval.zanella@linaro.org
State	Dropped
Headers	Mailing-List: contact libc-alpha-help@sourceware.org; run by ezmlm Precedence: bulk Sender: libc-alpha-owner@sourceware.org From: Adhemerval Zanella <adhemerval.zanella@linaro.org> To: libc-alpha@sourceware.org Subject: [PATCH] Fix i686 memchr overflow calculation (BZ#21182) Date: Tue, 14 Mar 2017 14:28:07 -0300 Message-Id: <1489512487-24860-1-git-send-email-adhemerval.zanella@linaro.org>

Message ID

1489512487-24860-1-git-send-email-adhemerval.zanella@linaro.org

State

Dropped

Headers

Mailing-List: contact libc-alpha-help@sourceware.org; run by ezmlm
Precedence: bulk
Sender: libc-alpha-owner@sourceware.org
From: Adhemerval Zanella <adhemerval.zanella@linaro.org>
To: libc-alpha@sourceware.org
Subject: [PATCH] Fix i686 memchr overflow calculation (BZ#21182)
Date: Tue, 14 Mar 2017 14:28:07 -0300
Message-Id: <1489512487-24860-1-git-send-email-adhemerval.zanella@linaro.org>

Commit Message

Adhemerval Zanella March 14, 2017, 5:28 p.m. UTC

  This patch fixes the regression added by 23d2770 for final address
overflow calculation.  The subtraction of the considered size (16)
at line 120 is at wrong place, for sizes less than 16 subsequent
overflow check will not take in consideration an invalid size (since
the subtraction will be negative).  Also, the lea instruction also
does not raise the carry flag (CF) that is used in subsequent jbe
to check for overflow.

The fix is to follow x86_64 logic from 3daef2c where the overflow
is first check and a sub instruction is issued.  In case of resulting
negative size, CF will be set by the sub instruction and a NULL
result will be returned.  The patch also add similar tests reported
in bug report.

Checked on i686-linux-gnu and x86_64-linux-gnu.

	[BZ# 21182]
	* string/test-memchr.c (do_test): Add BZ#21182 checks for address
	near end of a page.
	* sysdeps/i386/i686/multiarch/memchr-sse2.S (__memchr): Fix
	overflow calculation.
---
 ChangeLog                                 | 7 +++++++
 string/test-memchr.c                      | 6 ++++++
 sysdeps/i386/i686/multiarch/memchr-sse2.S | 2 +-
 3 files changed, 14 insertions(+), 1 deletion(-)

Comments

Wainer dos Santos Moschetta March 14, 2017, 7:48 p.m. UTC | #1

On 03/14/2017 02:28 PM, Adhemerval Zanella wrote:
> This patch fixes the regression added by 23d2770 for final address
> overflow calculation.  The subtraction of the considered size (16)
> at line 120 is at wrong place, for sizes less than 16 subsequent
> overflow check will not take in consideration an invalid size (since
> the subtraction will be negative).  Also, the lea instruction also
> does not raise the carry flag (CF) that is used in subsequent jbe
> to check for overflow.
>
> The fix is to follow x86_64 logic from 3daef2c where the overflow
> is first check and a sub instruction is issued.  In case of resulting
> negative size, CF will be set by the sub instruction and a NULL
> result will be returned.  The patch also add similar tests reported
> in bug report.
>
> Checked on i686-linux-gnu and x86_64-linux-gnu.
>
> 	[BZ# 21182]
> 	* string/test-memchr.c (do_test): Add BZ#21182 checks for address
> 	near end of a page.
> 	* sysdeps/i386/i686/multiarch/memchr-sse2.S (__memchr): Fix
> 	overflow calculation.
> ---
>  ChangeLog                                 | 7 +++++++
>  string/test-memchr.c                      | 6 ++++++
>  sysdeps/i386/i686/multiarch/memchr-sse2.S | 2 +-
>  3 files changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/string/test-memchr.c b/string/test-memchr.c
> index d64d10c..87a077b 100644
> --- a/string/test-memchr.c
> +++ b/string/test-memchr.c
> @@ -210,6 +210,12 @@ test_main (void)
>        do_test (0, i, i + 1, i + 1, 0);
>      }
>
> +  /* BZ#21182 - wrong overflow calculation for i686 implementation
> +     with address near end of the page.  */
> +  for (i = 2; i < 16; ++i)
> +    /* page_size is in fact getpagesize() * 2.  */
> +    do_test (page_size/2 - i, i, i, 1, 0x9B);

page_size / 2

> +
>    do_random_tests ();
>    return ret;
>  }
> diff --git a/sysdeps/i386/i686/multiarch/memchr-sse2.S b/sysdeps/i386/i686/multiarch/memchr-sse2.S
> index 910679c..e41f324 100644
> --- a/sysdeps/i386/i686/multiarch/memchr-sse2.S
> +++ b/sysdeps/i386/i686/multiarch/memchr-sse2.S
> @@ -117,7 +117,6 @@ L(crosscache):
>
>  # ifndef USE_AS_RAWMEMCHR
>  	jnz	L(match_case2_prolog1)
> -	lea	-16(%edx), %edx
>          /* Calculate the last acceptable address and check for possible
>             addition overflow by using satured math:
>             edx = ecx + edx
> @@ -125,6 +124,7 @@ L(crosscache):
>  	add	%ecx, %edx
>  	sbb	%eax, %eax
>  	or	%eax, %edx
> +	sub	$16, %edx
>  	jbe	L(return_null)
>  	lea	16(%edi), %edi
>  # else

Adhemerval Zanella March 28, 2017, 7:22 p.m. UTC | #2

Ping (with the page_size/2 fix included).

On 14/03/2017 14:28, Adhemerval Zanella wrote:
> This patch fixes the regression added by 23d2770 for final address
> overflow calculation.  The subtraction of the considered size (16)
> at line 120 is at wrong place, for sizes less than 16 subsequent
> overflow check will not take in consideration an invalid size (since
> the subtraction will be negative).  Also, the lea instruction also
> does not raise the carry flag (CF) that is used in subsequent jbe
> to check for overflow.
> 
> The fix is to follow x86_64 logic from 3daef2c where the overflow
> is first check and a sub instruction is issued.  In case of resulting
> negative size, CF will be set by the sub instruction and a NULL
> result will be returned.  The patch also add similar tests reported
> in bug report.
> 
> Checked on i686-linux-gnu and x86_64-linux-gnu.
> 
> 	[BZ# 21182]
> 	* string/test-memchr.c (do_test): Add BZ#21182 checks for address
> 	near end of a page.
> 	* sysdeps/i386/i686/multiarch/memchr-sse2.S (__memchr): Fix
> 	overflow calculation.
> ---
>  ChangeLog                                 | 7 +++++++
>  string/test-memchr.c                      | 6 ++++++
>  sysdeps/i386/i686/multiarch/memchr-sse2.S | 2 +-
>  3 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/string/test-memchr.c b/string/test-memchr.c
> index d64d10c..87a077b 100644
> --- a/string/test-memchr.c
> +++ b/string/test-memchr.c
> @@ -210,6 +210,12 @@ test_main (void)
>        do_test (0, i, i + 1, i + 1, 0);
>      }
>  
> +  /* BZ#21182 - wrong overflow calculation for i686 implementation
> +     with address near end of the page.  */
> +  for (i = 2; i < 16; ++i)
> +    /* page_size is in fact getpagesize() * 2.  */
> +    do_test (page_size/2 - i, i, i, 1, 0x9B);
> +
>    do_random_tests ();
>    return ret;
>  }
> diff --git a/sysdeps/i386/i686/multiarch/memchr-sse2.S b/sysdeps/i386/i686/multiarch/memchr-sse2.S
> index 910679c..e41f324 100644
> --- a/sysdeps/i386/i686/multiarch/memchr-sse2.S
> +++ b/sysdeps/i386/i686/multiarch/memchr-sse2.S
> @@ -117,7 +117,6 @@ L(crosscache):
>  
>  # ifndef USE_AS_RAWMEMCHR
>  	jnz	L(match_case2_prolog1)
> -	lea	-16(%edx), %edx
>          /* Calculate the last acceptable address and check for possible
>             addition overflow by using satured math:
>             edx = ecx + edx
> @@ -125,6 +124,7 @@ L(crosscache):
>  	add	%ecx, %edx
>  	sbb	%eax, %eax
>  	or	%eax, %edx
> +	sub	$16, %edx
>  	jbe	L(return_null)
>  	lea	16(%edi), %edi
>  # else
>

H.J. Lu March 28, 2017, 9:47 p.m. UTC | #3

On Tue, Mar 28, 2017 at 12:22 PM, Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
> Ping (with the page_size/2 fix included).
>
> On 14/03/2017 14:28, Adhemerval Zanella wrote:
>> This patch fixes the regression added by 23d2770 for final address
>> overflow calculation.  The subtraction of the considered size (16)
>> at line 120 is at wrong place, for sizes less than 16 subsequent
>> overflow check will not take in consideration an invalid size (since
>> the subtraction will be negative).  Also, the lea instruction also
>> does not raise the carry flag (CF) that is used in subsequent jbe
>> to check for overflow.
>>
>> The fix is to follow x86_64 logic from 3daef2c where the overflow
>> is first check and a sub instruction is issued.  In case of resulting
>> negative size, CF will be set by the sub instruction and a NULL
>> result will be returned.  The patch also add similar tests reported
>> in bug report.
>>
>> Checked on i686-linux-gnu and x86_64-linux-gnu.
>>
>>       [BZ# 21182]
>>       * string/test-memchr.c (do_test): Add BZ#21182 checks for address
>>       near end of a page.
>>       * sysdeps/i386/i686/multiarch/memchr-sse2.S (__memchr): Fix
>>       overflow calculation.
>> ---
>>  ChangeLog                                 | 7 +++++++
>>  string/test-memchr.c                      | 6 ++++++
>>  sysdeps/i386/i686/multiarch/memchr-sse2.S | 2 +-
>>  3 files changed, 14 insertions(+), 1 deletion(-)
>>
>> diff --git a/string/test-memchr.c b/string/test-memchr.c
>> index d64d10c..87a077b 100644
>> --- a/string/test-memchr.c
>> +++ b/string/test-memchr.c
>> @@ -210,6 +210,12 @@ test_main (void)
>>        do_test (0, i, i + 1, i + 1, 0);
>>      }
>>
>> +  /* BZ#21182 - wrong overflow calculation for i686 implementation
>> +     with address near end of the page.  */
>> +  for (i = 2; i < 16; ++i)
>> +    /* page_size is in fact getpagesize() * 2.  */
>> +    do_test (page_size/2 - i, i, i, 1, 0x9B);
>> +
>>    do_random_tests ();
>>    return ret;
>>  }
>> diff --git a/sysdeps/i386/i686/multiarch/memchr-sse2.S b/sysdeps/i386/i686/multiarch/memchr-sse2.S
>> index 910679c..e41f324 100644
>> --- a/sysdeps/i386/i686/multiarch/memchr-sse2.S
>> +++ b/sysdeps/i386/i686/multiarch/memchr-sse2.S
>> @@ -117,7 +117,6 @@ L(crosscache):
>>
>>  # ifndef USE_AS_RAWMEMCHR
>>       jnz     L(match_case2_prolog1)
>> -     lea     -16(%edx), %edx
>>          /* Calculate the last acceptable address and check for possible
>>             addition overflow by using satured math:
>>             edx = ecx + edx
>> @@ -125,6 +124,7 @@ L(crosscache):
>>       add     %ecx, %edx
>>       sbb     %eax, %eax
>>       or      %eax, %edx
>> +     sub     $16, %edx
>>       jbe     L(return_null)
>>       lea     16(%edi), %edi
>>  # else
>>

Looks good.

Thanks.

H.J. Lu May 19, 2017, 1:44 p.m. UTC | #4

On Tue, Mar 28, 2017 at 2:47 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Tue, Mar 28, 2017 at 12:22 PM, Adhemerval Zanella
> <adhemerval.zanella@linaro.org> wrote:
>> Ping (with the page_size/2 fix included).
>>
>> On 14/03/2017 14:28, Adhemerval Zanella wrote:
>>> This patch fixes the regression added by 23d2770 for final address
>>> overflow calculation.  The subtraction of the considered size (16)
>>> at line 120 is at wrong place, for sizes less than 16 subsequent
>>> overflow check will not take in consideration an invalid size (since
>>> the subtraction will be negative).  Also, the lea instruction also
>>> does not raise the carry flag (CF) that is used in subsequent jbe
>>> to check for overflow.
>>>
>>> The fix is to follow x86_64 logic from 3daef2c where the overflow
>>> is first check and a sub instruction is issued.  In case of resulting
>>> negative size, CF will be set by the sub instruction and a NULL
>>> result will be returned.  The patch also add similar tests reported
>>> in bug report.
>>>
>>> Checked on i686-linux-gnu and x86_64-linux-gnu.
>>>
>>>       [BZ# 21182]
>>>       * string/test-memchr.c (do_test): Add BZ#21182 checks for address
>>>       near end of a page.
>>>       * sysdeps/i386/i686/multiarch/memchr-sse2.S (__memchr): Fix
>>>       overflow calculation.
>>> ---
>>>  ChangeLog                                 | 7 +++++++
>>>  string/test-memchr.c                      | 6 ++++++
>>>  sysdeps/i386/i686/multiarch/memchr-sse2.S | 2 +-
>>>  3 files changed, 14 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/string/test-memchr.c b/string/test-memchr.c
>>> index d64d10c..87a077b 100644
>>> --- a/string/test-memchr.c
>>> +++ b/string/test-memchr.c
>>> @@ -210,6 +210,12 @@ test_main (void)
>>>        do_test (0, i, i + 1, i + 1, 0);
>>>      }
>>>
>>> +  /* BZ#21182 - wrong overflow calculation for i686 implementation
>>> +     with address near end of the page.  */
>>> +  for (i = 2; i < 16; ++i)
>>> +    /* page_size is in fact getpagesize() * 2.  */
>>> +    do_test (page_size/2 - i, i, i, 1, 0x9B);
>>> +
>>>    do_random_tests ();
>>>    return ret;
>>>  }
>>> diff --git a/sysdeps/i386/i686/multiarch/memchr-sse2.S b/sysdeps/i386/i686/multiarch/memchr-sse2.S
>>> index 910679c..e41f324 100644
>>> --- a/sysdeps/i386/i686/multiarch/memchr-sse2.S
>>> +++ b/sysdeps/i386/i686/multiarch/memchr-sse2.S
>>> @@ -117,7 +117,6 @@ L(crosscache):
>>>
>>>  # ifndef USE_AS_RAWMEMCHR
>>>       jnz     L(match_case2_prolog1)
>>> -     lea     -16(%edx), %edx
>>>          /* Calculate the last acceptable address and check for possible
>>>             addition overflow by using satured math:
>>>             edx = ecx + edx
>>> @@ -125,6 +124,7 @@ L(crosscache):
>>>       add     %ecx, %edx
>>>       sbb     %eax, %eax
>>>       or      %eax, %edx
>>> +     sub     $16, %edx
>>>       jbe     L(return_null)
>>>       lea     16(%edi), %edi
>>>  # else
>>>
>
> Looks good.
>
>

Just a thought.  Is this approach

        /* Calculate "edx + ecx - 16" by using "edx - (16 - ecx)"
           instead of "(edx + ecx) - 16" to void possible addition
           overflow.  */
        neg     %ecx
        add     $16, %ecx
        sub     %ecx, %edx
        jbe     L(return_null)

a little bit better?

Adhemerval Zanella May 19, 2017, 2:45 p.m. UTC | #5

On 19/05/2017 10:44, H.J. Lu wrote:
> On Tue, Mar 28, 2017 at 2:47 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Tue, Mar 28, 2017 at 12:22 PM, Adhemerval Zanella
>> <adhemerval.zanella@linaro.org> wrote:
>>> Ping (with the page_size/2 fix included).
>>>
>>> On 14/03/2017 14:28, Adhemerval Zanella wrote:
>>>> This patch fixes the regression added by 23d2770 for final address
>>>> overflow calculation.  The subtraction of the considered size (16)
>>>> at line 120 is at wrong place, for sizes less than 16 subsequent
>>>> overflow check will not take in consideration an invalid size (since
>>>> the subtraction will be negative).  Also, the lea instruction also
>>>> does not raise the carry flag (CF) that is used in subsequent jbe
>>>> to check for overflow.
>>>>
>>>> The fix is to follow x86_64 logic from 3daef2c where the overflow
>>>> is first check and a sub instruction is issued.  In case of resulting
>>>> negative size, CF will be set by the sub instruction and a NULL
>>>> result will be returned.  The patch also add similar tests reported
>>>> in bug report.
>>>>
>>>> Checked on i686-linux-gnu and x86_64-linux-gnu.
>>>>
>>>>       [BZ# 21182]
>>>>       * string/test-memchr.c (do_test): Add BZ#21182 checks for address
>>>>       near end of a page.
>>>>       * sysdeps/i386/i686/multiarch/memchr-sse2.S (__memchr): Fix
>>>>       overflow calculation.
>>>> ---
>>>>  ChangeLog                                 | 7 +++++++
>>>>  string/test-memchr.c                      | 6 ++++++
>>>>  sysdeps/i386/i686/multiarch/memchr-sse2.S | 2 +-
>>>>  3 files changed, 14 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/string/test-memchr.c b/string/test-memchr.c
>>>> index d64d10c..87a077b 100644
>>>> --- a/string/test-memchr.c
>>>> +++ b/string/test-memchr.c
>>>> @@ -210,6 +210,12 @@ test_main (void)
>>>>        do_test (0, i, i + 1, i + 1, 0);
>>>>      }
>>>>
>>>> +  /* BZ#21182 - wrong overflow calculation for i686 implementation
>>>> +     with address near end of the page.  */
>>>> +  for (i = 2; i < 16; ++i)
>>>> +    /* page_size is in fact getpagesize() * 2.  */
>>>> +    do_test (page_size/2 - i, i, i, 1, 0x9B);
>>>> +
>>>>    do_random_tests ();
>>>>    return ret;
>>>>  }
>>>> diff --git a/sysdeps/i386/i686/multiarch/memchr-sse2.S b/sysdeps/i386/i686/multiarch/memchr-sse2.S
>>>> index 910679c..e41f324 100644
>>>> --- a/sysdeps/i386/i686/multiarch/memchr-sse2.S
>>>> +++ b/sysdeps/i386/i686/multiarch/memchr-sse2.S
>>>> @@ -117,7 +117,6 @@ L(crosscache):
>>>>
>>>>  # ifndef USE_AS_RAWMEMCHR
>>>>       jnz     L(match_case2_prolog1)
>>>> -     lea     -16(%edx), %edx
>>>>          /* Calculate the last acceptable address and check for possible
>>>>             addition overflow by using satured math:
>>>>             edx = ecx + edx
>>>> @@ -125,6 +124,7 @@ L(crosscache):
>>>>       add     %ecx, %edx
>>>>       sbb     %eax, %eax
>>>>       or      %eax, %edx
>>>> +     sub     $16, %edx
>>>>       jbe     L(return_null)
>>>>       lea     16(%edi), %edi
>>>>  # else
>>>>
>>
>> Looks good.
>>
>>
> 
> Just a thought.  Is this approach
> 
>         /* Calculate "edx + ecx - 16" by using "edx - (16 - ecx)"
>            instead of "(edx + ecx) - 16" to void possible addition
>            overflow.  */
>         neg     %ecx
>         add     $16, %ecx
>         sub     %ecx, %edx
>         jbe     L(return_null)
> 
> a little bit better?

I do not have a strong preference here, although imho it is simpler to
understand why the saturated math would work here and it is the same
strategy used on other arch implementations.  May you might outline in
comment that ecx is always in the range of [0,64) so '-ecx + 16' can
possible underflow.  Is is this change just for micro-optimization?

Adhemerval Zanella May 19, 2017, 3:10 p.m. UTC | #6

On 19/05/2017 11:45, Adhemerval Zanella wrote:
> 
> 
> On 19/05/2017 10:44, H.J. Lu wrote:
>> On Tue, Mar 28, 2017 at 2:47 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> On Tue, Mar 28, 2017 at 12:22 PM, Adhemerval Zanella
>>> <adhemerval.zanella@linaro.org> wrote:
>>>> Ping (with the page_size/2 fix included).
>>>>
>>>> On 14/03/2017 14:28, Adhemerval Zanella wrote:
>>>>> This patch fixes the regression added by 23d2770 for final address
>>>>> overflow calculation.  The subtraction of the considered size (16)
>>>>> at line 120 is at wrong place, for sizes less than 16 subsequent
>>>>> overflow check will not take in consideration an invalid size (since
>>>>> the subtraction will be negative).  Also, the lea instruction also
>>>>> does not raise the carry flag (CF) that is used in subsequent jbe
>>>>> to check for overflow.
>>>>>
>>>>> The fix is to follow x86_64 logic from 3daef2c where the overflow
>>>>> is first check and a sub instruction is issued.  In case of resulting
>>>>> negative size, CF will be set by the sub instruction and a NULL
>>>>> result will be returned.  The patch also add similar tests reported
>>>>> in bug report.
>>>>>
>>>>> Checked on i686-linux-gnu and x86_64-linux-gnu.
>>>>>
>>>>>       [BZ# 21182]
>>>>>       * string/test-memchr.c (do_test): Add BZ#21182 checks for address
>>>>>       near end of a page.
>>>>>       * sysdeps/i386/i686/multiarch/memchr-sse2.S (__memchr): Fix
>>>>>       overflow calculation.
>>>>> ---
>>>>>  ChangeLog                                 | 7 +++++++
>>>>>  string/test-memchr.c                      | 6 ++++++
>>>>>  sysdeps/i386/i686/multiarch/memchr-sse2.S | 2 +-
>>>>>  3 files changed, 14 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/string/test-memchr.c b/string/test-memchr.c
>>>>> index d64d10c..87a077b 100644
>>>>> --- a/string/test-memchr.c
>>>>> +++ b/string/test-memchr.c
>>>>> @@ -210,6 +210,12 @@ test_main (void)
>>>>>        do_test (0, i, i + 1, i + 1, 0);
>>>>>      }
>>>>>
>>>>> +  /* BZ#21182 - wrong overflow calculation for i686 implementation
>>>>> +     with address near end of the page.  */
>>>>> +  for (i = 2; i < 16; ++i)
>>>>> +    /* page_size is in fact getpagesize() * 2.  */
>>>>> +    do_test (page_size/2 - i, i, i, 1, 0x9B);
>>>>> +
>>>>>    do_random_tests ();
>>>>>    return ret;
>>>>>  }
>>>>> diff --git a/sysdeps/i386/i686/multiarch/memchr-sse2.S b/sysdeps/i386/i686/multiarch/memchr-sse2.S
>>>>> index 910679c..e41f324 100644
>>>>> --- a/sysdeps/i386/i686/multiarch/memchr-sse2.S
>>>>> +++ b/sysdeps/i386/i686/multiarch/memchr-sse2.S
>>>>> @@ -117,7 +117,6 @@ L(crosscache):
>>>>>
>>>>>  # ifndef USE_AS_RAWMEMCHR
>>>>>       jnz     L(match_case2_prolog1)
>>>>> -     lea     -16(%edx), %edx
>>>>>          /* Calculate the last acceptable address and check for possible
>>>>>             addition overflow by using satured math:
>>>>>             edx = ecx + edx
>>>>> @@ -125,6 +124,7 @@ L(crosscache):
>>>>>       add     %ecx, %edx
>>>>>       sbb     %eax, %eax
>>>>>       or      %eax, %edx
>>>>> +     sub     $16, %edx
>>>>>       jbe     L(return_null)
>>>>>       lea     16(%edi), %edi
>>>>>  # else
>>>>>
>>>
>>> Looks good.
>>>
>>>
>>
>> Just a thought.  Is this approach
>>
>>         /* Calculate "edx + ecx - 16" by using "edx - (16 - ecx)"
>>            instead of "(edx + ecx) - 16" to void possible addition
>>            overflow.  */
>>         neg     %ecx
>>         add     $16, %ecx
>>         sub     %ecx, %edx
>>         jbe     L(return_null)
>>
>> a little bit better?
> 
> I do not have a strong preference here, although imho it is simpler to
> understand why the saturated math would work here and it is the same
> strategy used on other arch implementations.  May you might outline in
> comment that ecx is always in the range of [0,64) so '-ecx + 16' can
> possible underflow.  Is is this change just for micro-optimization?
> 

Sorry, my bad here.  I made some confusion, in fact what I meant was ecx
was indeed in range [0,16) (is the and instruction for alignment that 
uses 64) and your suggestion *can't* overflow.  I would recommend
you to just outline the possible value of 'edx' in comments.

diff mbox

Patch

diff --git a/string/test-memchr.c b/string/test-memchr.c
index d64d10c..87a077b 100644
--- a/string/test-memchr.c
+++ b/string/test-memchr.c
@@ -210,6 +210,12 @@  test_main (void)
       do_test (0, i, i + 1, i + 1, 0);
     }
 
+  /* BZ#21182 - wrong overflow calculation for i686 implementation
+     with address near end of the page.  */
+  for (i = 2; i < 16; ++i)
+    /* page_size is in fact getpagesize() * 2.  */
+    do_test (page_size/2 - i, i, i, 1, 0x9B);
+
   do_random_tests ();
   return ret;
 }
diff --git a/sysdeps/i386/i686/multiarch/memchr-sse2.S b/sysdeps/i386/i686/multiarch/memchr-sse2.S
index 910679c..e41f324 100644
--- a/sysdeps/i386/i686/multiarch/memchr-sse2.S
+++ b/sysdeps/i386/i686/multiarch/memchr-sse2.S
@@ -117,7 +117,6 @@  L(crosscache):
 
 # ifndef USE_AS_RAWMEMCHR
 	jnz	L(match_case2_prolog1)
-	lea	-16(%edx), %edx
         /* Calculate the last acceptable address and check for possible
            addition overflow by using satured math:
            edx = ecx + edx
@@ -125,6 +124,7 @@  L(crosscache):
 	add	%ecx, %edx
 	sbb	%eax, %eax
 	or	%eax, %edx
+	sub	$16, %edx
 	jbe	L(return_null)
 	lea	16(%edi), %edi
 # else