[PATCH] Add a way to bypass the PLT when calling getauxval

[PATCH] Add a way to bypass the PLT when calling getauxval

[Tulio Magno Quites Machado Filho]

Add __libc_getauxval as an internal libc symbol allowing internal libc
calls to bypass the PLT.

This is leaving the ABI as it was: getauxval continues to be exported as a
weak symbol and __getauxval as a global symbol.

2017-05-24  Tulio Magno Quites Machado Filho  <tuliom@linux.vnet.ibm.com>

	* include/sys/auxv.h: Add a prototype for __libc_getauxval.
	* misc/getauxval.c (__getauxval): Rename to __libc_getauxval and
	add the strong alias __getauxval to __libc_getauxval.
---
 include/sys/auxv.h | 2 ++
 misc/getauxval.c   | 6 ++++--
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/include/sys/auxv.h b/include/sys/auxv.h
index dede2c3..7b6ad3b 100644
--- a/include/sys/auxv.h
+++ b/include/sys/auxv.h
@@ -1 +1,3 @@
 #include <misc/sys/auxv.h>
+
+extern __typeof (getauxval) __libc_getauxval;
diff --git a/misc/getauxval.c b/misc/getauxval.c
index c83fbce..ff3e0ba 100644
--- a/misc/getauxval.c
+++ b/misc/getauxval.c
@@ -21,7 +21,8 @@
 
 
 unsigned long int
-__getauxval (unsigned long int type)
+attribute_hidden
+__libc_getauxval (unsigned long int type)
 {
 #ifdef HAVE_AUX_VECTOR
   ElfW(auxv_t) *p;
@@ -42,4 +43,5 @@ __getauxval (unsigned long int type)
   return 0;
 }
 
-weak_alias (__getauxval, getauxval)
+weak_alias (__libc_getauxval, getauxval)
+strong_alias (__libc_getauxval, __getauxval)
-- 
2.1.0

Re: [PATCH] Add a way to bypass the PLT when calling getauxval

[Joseph Myers]

On Wed, 24 May 2017, Tulio Magno Quites Machado Filho wrote:

> Add __libc_getauxval as an internal libc symbol allowing internal libc
> calls to bypass the PLT.

Why add this alias instead of simply doing libc_hidden_proto (__getauxval) 
/ libc_hidden_def (__getauxval)?

-- 
Joseph S. Myers
joseph@codesourcery.com

Re: [PATCH] Add a way to bypass the PLT when calling getauxval

[Tulio Magno Quites Machado Filho]

Joseph Myers <joseph@codesourcery.com> writes:

> On Wed, 24 May 2017, Tulio Magno Quites Machado Filho wrote:
>
>> Add __libc_getauxval as an internal libc symbol allowing internal libc
>> calls to bypass the PLT.
>
> Why add this alias instead of simply doing libc_hidden_proto (__getauxval) 
> / libc_hidden_def (__getauxval)?

Because I made a wrong assumption about those macros.
I'll fix my patch and re-submit.

Thanks!

-- 
Tulio Magno

[PATCHv2] Add a way to bypass the PLT when calling getauxval

[Tulio Magno Quites Machado Filho]

2017-05-25  Tulio Magno Quites Machado Filho  <tuliom@linux.vnet.ibm.com>

	* include/sys/auxv.h (__getauxval): Add a prototype and its
	libc_hidden_proto.
	* misc/getauxval.c (__getauxval): Use libc_hidden_def.
---
 include/sys/auxv.h | 7 +++++++
 misc/getauxval.c   | 1 +
 2 files changed, 8 insertions(+)

diff --git a/include/sys/auxv.h b/include/sys/auxv.h
index dede2c3..d46008e 100644
--- a/include/sys/auxv.h
+++ b/include/sys/auxv.h
@@ -1 +1,8 @@
 #include <misc/sys/auxv.h>
+
+#ifndef _ISOMAC
+
+extern __typeof(getauxval) __getauxval;
+libc_hidden_proto (__getauxval)
+
+#endif  /* !_ISOMAC */
diff --git a/misc/getauxval.c b/misc/getauxval.c
index c83fbce..14f4298 100644
--- a/misc/getauxval.c
+++ b/misc/getauxval.c
@@ -43,3 +43,4 @@ __getauxval (unsigned long int type)
 }
 
 weak_alias (__getauxval, getauxval)
+libc_hidden_def (__getauxval)
-- 
2.1.0

Re: [PATCHv2] Add a way to bypass the PLT when calling getauxval

[Tulio Magno Quites Machado Filho]

Ping!

Tulio Magno Quites Machado Filho <tuliom@linux.vnet.ibm.com> writes:

> 2017-05-25  Tulio Magno Quites Machado Filho  <tuliom@linux.vnet.ibm.com>
>
> 	* include/sys/auxv.h (__getauxval): Add a prototype and its
> 	libc_hidden_proto.
> 	* misc/getauxval.c (__getauxval): Use libc_hidden_def.
> ---
>  include/sys/auxv.h | 7 +++++++
>  misc/getauxval.c   | 1 +
>  2 files changed, 8 insertions(+)
>
> diff --git a/include/sys/auxv.h b/include/sys/auxv.h
> index dede2c3..d46008e 100644
> --- a/include/sys/auxv.h
> +++ b/include/sys/auxv.h
> @@ -1 +1,8 @@
>  #include <misc/sys/auxv.h>
> +
> +#ifndef _ISOMAC
> +
> +extern __typeof(getauxval) __getauxval;
> +libc_hidden_proto (__getauxval)
> +
> +#endif  /* !_ISOMAC */
> diff --git a/misc/getauxval.c b/misc/getauxval.c
> index c83fbce..14f4298 100644
> --- a/misc/getauxval.c
> +++ b/misc/getauxval.c
> @@ -43,3 +43,4 @@ __getauxval (unsigned long int type)
>  }
>
>  weak_alias (__getauxval, getauxval)
> +libc_hidden_def (__getauxval)
> -- 
> 2.1.0
>

-- 
Tulio Magno

Re: [PATCHv2] Add a way to bypass the PLT when calling getauxval

[H.J. Lu]

On Fri, Jun 9, 2017 at 6:13 AM, Tulio Magno Quites Machado Filho
<tuliom@linux.vnet.ibm.com> wrote:
> Ping!
>
> Tulio Magno Quites Machado Filho <tuliom@linux.vnet.ibm.com> writes:
>
>> 2017-05-25  Tulio Magno Quites Machado Filho  <tuliom@linux.vnet.ibm.com>
>>
>>       * include/sys/auxv.h (__getauxval): Add a prototype and its
>>       libc_hidden_proto.
>>       * misc/getauxval.c (__getauxval): Use libc_hidden_def.
>> ---
>>  include/sys/auxv.h | 7 +++++++
>>  misc/getauxval.c   | 1 +
>>  2 files changed, 8 insertions(+)
>>
>> diff --git a/include/sys/auxv.h b/include/sys/auxv.h
>> index dede2c3..d46008e 100644
>> --- a/include/sys/auxv.h
>> +++ b/include/sys/auxv.h
>> @@ -1 +1,8 @@
>>  #include <misc/sys/auxv.h>
>> +
>> +#ifndef _ISOMAC
>> +
>> +extern __typeof(getauxval) __getauxval;
>> +libc_hidden_proto (__getauxval)
>> +
>> +#endif  /* !_ISOMAC */
>> diff --git a/misc/getauxval.c b/misc/getauxval.c
>> index c83fbce..14f4298 100644
>> --- a/misc/getauxval.c
>> +++ b/misc/getauxval.c
>> @@ -43,3 +43,4 @@ __getauxval (unsigned long int type)
>>  }
>>
>>  weak_alias (__getauxval, getauxval)
>> +libc_hidden_def (__getauxval)
>> --
>> 2.1.0
>>

Which one is better?

https://sourceware.org/ml/libc-alpha/2017-06/msg00332.html

-- 
H.J.

Re: [PATCHv2] Add a way to bypass the PLT when calling getauxval

[Florian Weimer]

"Tulio Magno Quites Machado Filho" <tuliom@linux.vnet.ibm.com> writes:

> +extern __typeof(getauxval) __getauxval;

Missing space before paren.

> diff --git a/misc/getauxval.c b/misc/getauxval.c
> index c83fbce..14f4298 100644
> --- a/misc/getauxval.c
> +++ b/misc/getauxval.c
> @@ -43,3 +43,4 @@ __getauxval (unsigned long int type)
>  }
>  
>  weak_alias (__getauxval, getauxval)
> +libc_hidden_def (__getauxval)

Looks good.

Thanks,
Florian

Re: [PATCHv2] Add a way to bypass the PLT when calling getauxval

[Florian Weimer]

"H.J. Lu" <hjl.tools@gmail.com> writes:

>>> diff --git a/misc/getauxval.c b/misc/getauxval.c
>>> index c83fbce..14f4298 100644
>>> --- a/misc/getauxval.c
>>> +++ b/misc/getauxval.c
>>> @@ -43,3 +43,4 @@ __getauxval (unsigned long int type)
>>>  }
>>>
>>>  weak_alias (__getauxval, getauxval)
>>> +libc_hidden_def (__getauxval)
>>> --
>>> 2.1.0
>>>
>
> Which one is better?
>
> https://sourceware.org/ml/libc-alpha/2017-06/msg00332.html

Tulio's patch is better because getauxval references could lead to
namespace violations.

Thanks,
Florian

Re: [PATCHv2] Add a way to bypass the PLT when calling getauxval

[Tulio Magno Quites Machado Filho]

Florian Weimer <fweimer@redhat.com> writes:

> "Tulio Magno Quites Machado Filho" <tuliom@linux.vnet.ibm.com> writes:
>
>> +extern __typeof(getauxval) __getauxval;
>
> Missing space before paren.

Fixed locally.

>> diff --git a/misc/getauxval.c b/misc/getauxval.c
>> index c83fbce..14f4298 100644
>> --- a/misc/getauxval.c
>> +++ b/misc/getauxval.c
>> @@ -43,3 +43,4 @@ __getauxval (unsigned long int type)
>>  }
>>  
>>  weak_alias (__getauxval, getauxval)
>> +libc_hidden_def (__getauxval)
>
> Looks good.

I'll push this patch with the fix.

Thanks!

-- 
Tulio Magno