diff mbox series

xfs: mark sb_fname as nonstring

Message ID 20180525151421.2317292-1-arnd@arndb.de
State New
Headers show
Series xfs: mark sb_fname as nonstring | expand

Commit Message

Arnd Bergmann May 25, 2018, 3:14 p.m. UTC
gcc-8 reports two warnings for the newly added getlabel/setlabel code:

fs/xfs/xfs_ioctl.c: In function 'xfs_ioc_getlabel':
fs/xfs/xfs_ioctl.c:1822:38: error: argument to 'sizeof' in 'strncpy' call is the same expression as the source; did you mean to use the size of the destination? [-Werror=sizeof-pointer-memaccess]
  strncpy(label, sbp->sb_fname, sizeof(sbp->sb_fname));
                                      ^
In function 'strncpy',
    inlined from 'xfs_ioc_setlabel' at /git/arm-soc/fs/xfs/xfs_ioctl.c:1863:2,
    inlined from 'xfs_file_ioctl' at /git/arm-soc/fs/xfs/xfs_ioctl.c:1918:10:
include/linux/string.h:254:9: error: '__builtin_strncpy' output may be truncated copying 12 bytes from a string of length 12 [-Werror=stringop-truncation]
  return __builtin_strncpy(p, q, size);

In both cases, part of the problem is that one of the strncpy()
arguments is a fixed-length character array with zero-padding rather
than a zero-terminated string. In the first one case, we also get an
odd warning about sizeof-pointer-memaccess, which doesn't seem right
(the sizeof is for an array that happens to be the same as the second
strncpy argument).

To work around the bogus warning, I use a plain 'XFSLABEL_MAX' for
the strncpy() length when copying the label in getlabel. For setlabel(),
using memcpy() with the correct length that is already known avoids
the second warning and is slightly simpler.

In a related issue, it appears that we accidentally skip the trailing
\0 when copying a 12-character label back to user space in getlabel().
Using the correct sizeof() argument here copies the extra character.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85602
Fixes: f7664b31975b ("xfs: implement online get/set fs label")
Cc: Eric Sandeen <sandeen@redhat.com>
Cc: Martin Sebor <msebor@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

---
 fs/xfs/xfs_ioctl.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

-- 
2.9.0

Comments

Christoph Hellwig May 25, 2018, 4:52 p.m. UTC | #1
> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c

> index 84fbf164cbc3..eb79f2bc4dcc 100644

> --- a/fs/xfs/xfs_ioctl.c

> +++ b/fs/xfs/xfs_ioctl.c

> @@ -1819,12 +1819,12 @@ xfs_ioc_getlabel(

>  	BUILD_BUG_ON(sizeof(sbp->sb_fname) > FSLABEL_MAX);

>  

>  	spin_lock(&mp->m_sb_lock);

> -	strncpy(label, sbp->sb_fname, sizeof(sbp->sb_fname));

> +	strncpy(label, sbp->sb_fname, XFSLABEL_MAX);

>  	spin_unlock(&mp->m_sb_lock);


Hmm, shouldn't we just do a memcpy here?

Also given that the kernel never even looks at sb_fname maybe
we can turn into an array of unsigned chars to escape those string
warnings?
Eric Sandeen May 25, 2018, 4:53 p.m. UTC | #2
On 5/25/18 10:14 AM, Arnd Bergmann wrote:
> gcc-8 reports two warnings for the newly added getlabel/setlabel code:


Thanks for catching these.

The patch summary confuses me, what does "mark sb_fname as nonstring"
mean in the context of the actual patch?

I hate strings in C!  ;)

> fs/xfs/xfs_ioctl.c: In function 'xfs_ioc_getlabel':

> fs/xfs/xfs_ioctl.c:1822:38: error: argument to 'sizeof' in 'strncpy' call is the same expression as the source; did you mean to use the size of the destination? [-Werror=sizeof-pointer-memaccess]

>    strncpy(label, sbp->sb_fname, sizeof(sbp->sb_fname));                                         ^

o_O hrpmh.

> In function 'strncpy',

>      inlined from 'xfs_ioc_setlabel' at /git/arm-soc/fs/xfs/xfs_ioctl.c:1863:2,

>      inlined from 'xfs_file_ioctl' at /git/arm-soc/fs/xfs/xfs_ioctl.c:1918:10:

> include/linux/string.h:254:9: error: '__builtin_strncpy' output may be truncated copying 12 bytes from a string of length 12 [-Werror=stringop-truncation]

>    return __builtin_strncpy(p, q, size);

> 

> In both cases, part of the problem is that one of the strncpy()

> arguments is a fixed-length character array with zero-padding rather

> than a zero-terminated string. In the first one case, we also get an

> odd warning about sizeof-pointer-memaccess, which doesn't seem right

> (the sizeof is for an array that happens to be the same as the second

> strncpy argument).

> 

> To work around the bogus warning, I use a plain 'XFSLABEL_MAX' for

> the strncpy() length when copying the label in getlabel. For setlabel(),

> using memcpy() with the correct length that is already known avoids

> the second warning and is slightly simpler.

> 

> In a related issue, it appears that we accidentally skip the trailing

> \0 when copying a 12-character label back to user space in getlabel().

> Using the correct sizeof() argument here copies the extra character.


Oops, you are correct.  Sigh, I wonder why testing didn't see that.

> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85602

> Fixes: f7664b31975b ("xfs: implement online get/set fs label")

> Cc: Eric Sandeen <sandeen@redhat.com>

> Cc: Martin Sebor <msebor@gmail.com>

> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

> ---

>   fs/xfs/xfs_ioctl.c | 6 +++---

>   1 file changed, 3 insertions(+), 3 deletions(-)

> 

> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c

> index 84fbf164cbc3..eb79f2bc4dcc 100644

> --- a/fs/xfs/xfs_ioctl.c

> +++ b/fs/xfs/xfs_ioctl.c

> @@ -1819,12 +1819,12 @@ xfs_ioc_getlabel(

>   	BUILD_BUG_ON(sizeof(sbp->sb_fname) > FSLABEL_MAX);

>   

>   	spin_lock(&mp->m_sb_lock);

> -	strncpy(label, sbp->sb_fname, sizeof(sbp->sb_fname));

> +	strncpy(label, sbp->sb_fname, XFSLABEL_MAX);


ok

>   	spin_unlock(&mp->m_sb_lock);

>   

>   	/* xfs on-disk label is 12 chars, be sure we send a null to user */

>   	label[XFSLABEL_MAX] = '\0';

> -	if (copy_to_user(user_label, label, sizeof(sbp->sb_fname)))

> +	if (copy_to_user(user_label, label, sizeof(label)))



ok.  (odd how this is ok for copy_to_user but not for strncpy above) :)

>   		return -EFAULT;

>   	return 0;

>   }

> @@ -1860,7 +1860,7 @@ xfs_ioc_setlabel(

>   

>   	spin_lock(&mp->m_sb_lock);

>   	memset(sbp->sb_fname, 0, sizeof(sbp->sb_fname));

> -	strncpy(sbp->sb_fname, label, sizeof(sbp->sb_fname));

> +	memcpy(sbp->sb_fname, label, len);


Hm but len = strnlen(label, XFSLABEL_MAX + 1);
which could be one longer than sbp->sb_fname, no?

>   	spin_unlock(&mp->m_sb_lock);

>   

>   	/*

>
Arnd Bergmann May 25, 2018, 8:16 p.m. UTC | #3
On Fri, May 25, 2018 at 6:53 PM, Eric Sandeen <sandeen@redhat.com> wrote:
> On 5/25/18 10:14 AM, Arnd Bergmann wrote:

>>

>> gcc-8 reports two warnings for the newly added getlabel/setlabel code:

>

>

> Thanks for catching these.

>

> The patch summary confuses me, what does "mark sb_fname as nonstring"

> mean in the context of the actual patch?


My mistake. I tried a few different approaches and ended up using the
subject line from an earlier version with a later patch.

The 'nonstring' annotation is a variable attribute that gets gcc-8
to shut up about -Wstringop-truncation warnings, and is intended
to mark those character arrays that are not expected to be
null-terminated but still used with strncpy().

>>         spin_unlock(&mp->m_sb_lock);

>>         /* xfs on-disk label is 12 chars, be sure we send a null to user

>> */

>>         label[XFSLABEL_MAX] = '\0';

>> -       if (copy_to_user(user_label, label, sizeof(sbp->sb_fname)))

>> +       if (copy_to_user(user_label, label, sizeof(label)))

>

>

>

> ok.  (odd how this is ok for copy_to_user but not for strncpy above) :)


No idea. Maybe the gcc bug only happens with struct members but
not local variables?

>>                 return -EFAULT;

>>         return 0;

>>   }

>> @@ -1860,7 +1860,7 @@ xfs_ioc_setlabel(

>>         spin_lock(&mp->m_sb_lock);

>>         memset(sbp->sb_fname, 0, sizeof(sbp->sb_fname));

>> -       strncpy(sbp->sb_fname, label, sizeof(sbp->sb_fname));

>> +       memcpy(sbp->sb_fname, label, len);

>

>

> Hm but len = strnlen(label, XFSLABEL_MAX + 1);

> which could be one longer than sbp->sb_fname, no?


We have an explicit check for that, so I think it's ok:

        if (len > sizeof(sbp->sb_fname))
                return -EINVAL;

       Arnd
Arnd Bergmann May 25, 2018, 8:18 p.m. UTC | #4
On Fri, May 25, 2018 at 6:52 PM, Christoph Hellwig <hch@infradead.org> wrote:
>> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c

>> index 84fbf164cbc3..eb79f2bc4dcc 100644

>> --- a/fs/xfs/xfs_ioctl.c

>> +++ b/fs/xfs/xfs_ioctl.c

>> @@ -1819,12 +1819,12 @@ xfs_ioc_getlabel(

>>       BUILD_BUG_ON(sizeof(sbp->sb_fname) > FSLABEL_MAX);

>>

>>       spin_lock(&mp->m_sb_lock);

>> -     strncpy(label, sbp->sb_fname, sizeof(sbp->sb_fname));

>> +     strncpy(label, sbp->sb_fname, XFSLABEL_MAX);

>>       spin_unlock(&mp->m_sb_lock);

>

> Hmm, shouldn't we just do a memcpy here?


I thought about that as well, but decided that strncpy()'s zero-padding
is better here than padding with potentially random contents of the user
space stack.

> Also given that the kernel never even looks at sb_fname maybe

> we can turn into an array of unsigned chars to escape those string

> warnings?


I don't think that makes a difference to gcc.

        Arnd
Eric Sandeen May 25, 2018, 8:21 p.m. UTC | #5
On 5/25/18 3:16 PM, Arnd Bergmann wrote:

> On Fri, May 25, 2018 at 6:53 PM, Eric Sandeen <sandeen@redhat.com> wrote:

>> On 5/25/18 10:14 AM, Arnd Bergmann wrote:

...

>>> @@ -1860,7 +1860,7 @@ xfs_ioc_setlabel(

>>>          spin_lock(&mp->m_sb_lock);

>>>          memset(sbp->sb_fname, 0, sizeof(sbp->sb_fname));

>>> -       strncpy(sbp->sb_fname, label, sizeof(sbp->sb_fname));

>>> +       memcpy(sbp->sb_fname, label, len);

>>

>>

>> Hm but len = strnlen(label, XFSLABEL_MAX + 1);

>> which could be one longer than sbp->sb_fname, no?

> 

> We have an explicit check for that, so I think it's ok:

> 

>          if (len > sizeof(sbp->sb_fname))

>                  return -EINVAL;


Ah so we do; I wrote this at least 2 weeks ago, you're asking a lot for
me to remember it.  (or to even read it, apparently).   ;)

Thanks,
-Eric
Eric Sandeen June 5, 2018, 6:44 p.m. UTC | #6
On 5/25/18 10:14 AM, Arnd Bergmann wrote:
> gcc-8 reports two warnings for the newly added getlabel/setlabel code:

> 

> fs/xfs/xfs_ioctl.c: In function 'xfs_ioc_getlabel':

> fs/xfs/xfs_ioctl.c:1822:38: error: argument to 'sizeof' in 'strncpy' call is the same expression as the source; did you mean to use the size of the destination? [-Werror=sizeof-pointer-memaccess]

>   strncpy(label, sbp->sb_fname, sizeof(sbp->sb_fname));

>                                       ^

> In function 'strncpy',

>     inlined from 'xfs_ioc_setlabel' at /git/arm-soc/fs/xfs/xfs_ioctl.c:1863:2,

>     inlined from 'xfs_file_ioctl' at /git/arm-soc/fs/xfs/xfs_ioctl.c:1918:10:

> include/linux/string.h:254:9: error: '__builtin_strncpy' output may be truncated copying 12 bytes from a string of length 12 [-Werror=stringop-truncation]

>   return __builtin_strncpy(p, q, size);

> 

> In both cases, part of the problem is that one of the strncpy()

> arguments is a fixed-length character array with zero-padding rather

> than a zero-terminated string. In the first one case, we also get an

> odd warning about sizeof-pointer-memaccess, which doesn't seem right

> (the sizeof is for an array that happens to be the same as the second

> strncpy argument).

> 

> To work around the bogus warning, I use a plain 'XFSLABEL_MAX' for

> the strncpy() length when copying the label in getlabel. For setlabel(),

> using memcpy() with the correct length that is already known avoids

> the second warning and is slightly simpler.

> 

> In a related issue, it appears that we accidentally skip the trailing

> \0 when copying a 12-character label back to user space in getlabel().

> Using the correct sizeof() argument here copies the extra character.

> 

> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85602

> Fixes: f7664b31975b ("xfs: implement online get/set fs label")

> Cc: Eric Sandeen <sandeen@redhat.com>

> Cc: Martin Sebor <msebor@gmail.com>

> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

> ---

>  fs/xfs/xfs_ioctl.c | 6 +++---

>  1 file changed, 3 insertions(+), 3 deletions(-)

> 

> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c

> index 84fbf164cbc3..eb79f2bc4dcc 100644

> --- a/fs/xfs/xfs_ioctl.c

> +++ b/fs/xfs/xfs_ioctl.c

> @@ -1819,12 +1819,12 @@ xfs_ioc_getlabel(

>  	BUILD_BUG_ON(sizeof(sbp->sb_fname) > FSLABEL_MAX);

>  

>  	spin_lock(&mp->m_sb_lock);

> -	strncpy(label, sbp->sb_fname, sizeof(sbp->sb_fname));

> +	strncpy(label, sbp->sb_fname, XFSLABEL_MAX);

>  	spin_unlock(&mp->m_sb_lock);

>  

>  	/* xfs on-disk label is 12 chars, be sure we send a null to user */

>  	label[XFSLABEL_MAX] = '\0';

> -	if (copy_to_user(user_label, label, sizeof(sbp->sb_fname)))

> +	if (copy_to_user(user_label, label, sizeof(label)))


I /think/ this also runs the risk of copying out stack memory.

I'll send another proposal based on this with slight modifications.

>  		return -EFAULT;

>  	return 0;

>  }

> @@ -1860,7 +1860,7 @@ xfs_ioc_setlabel(

>  

>  	spin_lock(&mp->m_sb_lock);

>  	memset(sbp->sb_fname, 0, sizeof(sbp->sb_fname));

> -	strncpy(sbp->sb_fname, label, sizeof(sbp->sb_fname));

> +	memcpy(sbp->sb_fname, label, len);

>  	spin_unlock(&mp->m_sb_lock);

>  

>  	/*

>
Arnd Bergmann June 5, 2018, 9:23 p.m. UTC | #7
On Tue, Jun 5, 2018 at 8:44 PM, Eric Sandeen <sandeen@sandeen.net> wrote:
> On 5/25/18 10:14 AM, Arnd Bergmann wrote:


>> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c

>> index 84fbf164cbc3..eb79f2bc4dcc 100644

>> --- a/fs/xfs/xfs_ioctl.c

>> +++ b/fs/xfs/xfs_ioctl.c

>> @@ -1819,12 +1819,12 @@ xfs_ioc_getlabel(

>>       BUILD_BUG_ON(sizeof(sbp->sb_fname) > FSLABEL_MAX);

>>

>>       spin_lock(&mp->m_sb_lock);

>> -     strncpy(label, sbp->sb_fname, sizeof(sbp->sb_fname));

>> +     strncpy(label, sbp->sb_fname, XFSLABEL_MAX);

>>       spin_unlock(&mp->m_sb_lock);

>>

>>       /* xfs on-disk label is 12 chars, be sure we send a null to user */

>>       label[XFSLABEL_MAX] = '\0';

>> -     if (copy_to_user(user_label, label, sizeof(sbp->sb_fname)))

>> +     if (copy_to_user(user_label, label, sizeof(label)))

>

> I /think/ this also runs the risk of copying out stack memory.

>

> I'll send another proposal based on this with slight modifications.


I assumed it's safe since the earlier strncpy() pads the local 'label'
with zero bytes up the XFSLABEL_MAX, and the last byte
is explicitly set to guarantee zero padding.

Using strlcpy() or strscpy() would guarantee a zero-terminated
string without the explicit ='\0' assignment but would risk
the data leak you were probably thinking of.

       Arnd
Eric Sandeen June 5, 2018, 9:51 p.m. UTC | #8
On 6/5/18 4:23 PM, Arnd Bergmann wrote:
> On Tue, Jun 5, 2018 at 8:44 PM, Eric Sandeen <sandeen@sandeen.net> wrote:

>> On 5/25/18 10:14 AM, Arnd Bergmann wrote:

> 

>>> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c

>>> index 84fbf164cbc3..eb79f2bc4dcc 100644

>>> --- a/fs/xfs/xfs_ioctl.c

>>> +++ b/fs/xfs/xfs_ioctl.c

>>> @@ -1819,12 +1819,12 @@ xfs_ioc_getlabel(

>>>       BUILD_BUG_ON(sizeof(sbp->sb_fname) > FSLABEL_MAX);

>>>

>>>       spin_lock(&mp->m_sb_lock);

>>> -     strncpy(label, sbp->sb_fname, sizeof(sbp->sb_fname));

>>> +     strncpy(label, sbp->sb_fname, XFSLABEL_MAX);

>>>       spin_unlock(&mp->m_sb_lock);

>>>

>>>       /* xfs on-disk label is 12 chars, be sure we send a null to user */

>>>       label[XFSLABEL_MAX] = '\0';

>>> -     if (copy_to_user(user_label, label, sizeof(sbp->sb_fname)))

>>> +     if (copy_to_user(user_label, label, sizeof(label)))

>>

>> I /think/ this also runs the risk of copying out stack memory.

>>

>> I'll send another proposal based on this with slight modifications.

> 

> I assumed it's safe since the earlier strncpy() pads the local 'label'

> with zero bytes up the XFSLABEL_MAX, and the last byte

> is explicitly set to guarantee zero padding.


Ah you are right, I forgot that strncpy pads.  Did I mention that I hate
C strings... o_O

In that case I suppose your original patch is fine, if you'd like to fix
the "mark as nonstring" subject.

Thanks,
-Eric
diff mbox series

Patch

diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index 84fbf164cbc3..eb79f2bc4dcc 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c
@@ -1819,12 +1819,12 @@  xfs_ioc_getlabel(
 	BUILD_BUG_ON(sizeof(sbp->sb_fname) > FSLABEL_MAX);
 
 	spin_lock(&mp->m_sb_lock);
-	strncpy(label, sbp->sb_fname, sizeof(sbp->sb_fname));
+	strncpy(label, sbp->sb_fname, XFSLABEL_MAX);
 	spin_unlock(&mp->m_sb_lock);
 
 	/* xfs on-disk label is 12 chars, be sure we send a null to user */
 	label[XFSLABEL_MAX] = '\0';
-	if (copy_to_user(user_label, label, sizeof(sbp->sb_fname)))
+	if (copy_to_user(user_label, label, sizeof(label)))
 		return -EFAULT;
 	return 0;
 }
@@ -1860,7 +1860,7 @@  xfs_ioc_setlabel(
 
 	spin_lock(&mp->m_sb_lock);
 	memset(sbp->sb_fname, 0, sizeof(sbp->sb_fname));
-	strncpy(sbp->sb_fname, label, sizeof(sbp->sb_fname));
+	memcpy(sbp->sb_fname, label, len);
 	spin_unlock(&mp->m_sb_lock);
 
 	/*