[RFC v2] linux: use __fd_to_filename helper function instead of snprintf.

[RFC v2] linux: use __fd_to_filename helper function instead of snprintf.

[Érico Nogueira]

Change made to fchmodat and fexecve. There are tests using xasprintf
instead of this helper as well, but this commit doesn't touch them.
---

Differences from v1:
- actually builds
- doesn't use an intermediary buf variable

 sysdeps/unix/sysv/linux/fchmodat.c | 13 +++----------
 sysdeps/unix/sysv/linux/fexecve.c  | 10 ++++------
 2 files changed, 7 insertions(+), 16 deletions(-)

diff --git a/sysdeps/unix/sysv/linux/fchmodat.c b/sysdeps/unix/sysv/linux/fchmodat.c
index f264f0c09d..5bd1eb96a5 100644
--- a/sysdeps/unix/sysv/linux/fchmodat.c
+++ b/sysdeps/unix/sysv/linux/fchmodat.c
@@ -18,6 +18,7 @@
 
 #include <errno.h>
 #include <fcntl.h>
+#include <fd_to_filename.h>
 #include <not-cancel.h>
 #include <stdio.h>
 #include <sys/stat.h>
@@ -69,16 +70,8 @@ fchmodat (int fd, const char *file, mode_t mode, int flag)
 
       /* For most file systems, fchmod does not operate on O_PATH
 	 descriptors, so go through /proc.  */
-      char buf[32];
-      if (__snprintf (buf, sizeof (buf), "/proc/self/fd/%d", pathfd) < 0)
-	{
-	  /* This also may report strange error codes to the caller
-	     (although snprintf really should not fail).  */
-	  __close_nocancel (pathfd);
-	  return -1;
-	}
-
-      int ret = __chmod (buf, mode);
+      struct fd_to_filename filename;
+      int ret = __chmod (__fd_to_filename (pathfd, &filename), mode);
       if (ret != 0)
 	{
 	  if (errno == ENOENT)
diff --git a/sysdeps/unix/sysv/linux/fexecve.c b/sysdeps/unix/sysv/linux/fexecve.c
index f37c245396..df25c2acb8 100644
--- a/sysdeps/unix/sysv/linux/fexecve.c
+++ b/sysdeps/unix/sysv/linux/fexecve.c
@@ -22,6 +22,7 @@
 #include <fcntl.h>
 #include <sys/stat.h>
 
+#include <fd_to_filename.h>
 #include <sysdep.h>
 #include <sys/syscall.h>
 #include <kernel-features.h>
@@ -49,12 +50,9 @@ fexecve (int fd, char *const argv[], char *const envp[])
 
 #ifndef __ASSUME_EXECVEAT
   /* We use the /proc filesystem to get the information.  If it is not
-     mounted we fail.  */
-  char buf[sizeof "/proc/self/fd/" + sizeof (int) * 3];
-  __snprintf (buf, sizeof (buf), "/proc/self/fd/%d", fd);
-
-  /* We do not need the return value.  */
-  __execve (buf, argv, envp);
+     mounted we fail.  We do not need the return value.  */
+  struct fd_to_filename filename;
+  __execve (__fd_to_filename (fd, &filename), argv, envp);
 
   int save = errno;
 
-- 
2.31.1

Re: [RFC v2] linux: use __fd_to_filename helper function instead of snprintf.

[Adhemerval Zanella]

On 27/04/2021 10:09, Érico Nogueira via Libc-alpha wrote:
> Change made to fchmodat and fexecve. There are tests using xasprintf
> instead of this helper as well, but this commit doesn't touch them.

To adapt the tests it would require either to build it as internal/static
or add fd_to_filename to libsupport.  I am not this is really an improvement. 

LGTM, thanks.  I will commit this for you.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> ---
> 
> Differences from v1:
> - actually builds
> - doesn't use an intermediary buf variable
> 
>  sysdeps/unix/sysv/linux/fchmodat.c | 13 +++----------
>  sysdeps/unix/sysv/linux/fexecve.c  | 10 ++++------
>  2 files changed, 7 insertions(+), 16 deletions(-)
> 
> diff --git a/sysdeps/unix/sysv/linux/fchmodat.c b/sysdeps/unix/sysv/linux/fchmodat.c
> index f264f0c09d..5bd1eb96a5 100644
> --- a/sysdeps/unix/sysv/linux/fchmodat.c
> +++ b/sysdeps/unix/sysv/linux/fchmodat.c
> @@ -18,6 +18,7 @@
>  
>  #include <errno.h>
>  #include <fcntl.h>
> +#include <fd_to_filename.h>
>  #include <not-cancel.h>
>  #include <stdio.h>
>  #include <sys/stat.h>
> @@ -69,16 +70,8 @@ fchmodat (int fd, const char *file, mode_t mode, int flag)
>  
>        /* For most file systems, fchmod does not operate on O_PATH
>  	 descriptors, so go through /proc.  */
> -      char buf[32];
> -      if (__snprintf (buf, sizeof (buf), "/proc/self/fd/%d", pathfd) < 0)
> -	{
> -	  /* This also may report strange error codes to the caller
> -	     (although snprintf really should not fail).  */
> -	  __close_nocancel (pathfd);
> -	  return -1;
> -	}
> -
> -      int ret = __chmod (buf, mode);
> +      struct fd_to_filename filename;
> +      int ret = __chmod (__fd_to_filename (pathfd, &filename), mode);
>        if (ret != 0)
>  	{
>  	  if (errno == ENOENT)
> diff --git a/sysdeps/unix/sysv/linux/fexecve.c b/sysdeps/unix/sysv/linux/fexecve.c
> index f37c245396..df25c2acb8 100644
> --- a/sysdeps/unix/sysv/linux/fexecve.c
> +++ b/sysdeps/unix/sysv/linux/fexecve.c
> @@ -22,6 +22,7 @@
>  #include <fcntl.h>
>  #include <sys/stat.h>
>  
> +#include <fd_to_filename.h>
>  #include <sysdep.h>
>  #include <sys/syscall.h>
>  #include <kernel-features.h>
> @@ -49,12 +50,9 @@ fexecve (int fd, char *const argv[], char *const envp[])
>  
>  #ifndef __ASSUME_EXECVEAT
>    /* We use the /proc filesystem to get the information.  If it is not
> -     mounted we fail.  */
> -  char buf[sizeof "/proc/self/fd/" + sizeof (int) * 3];
> -  __snprintf (buf, sizeof (buf), "/proc/self/fd/%d", fd);
> -
> -  /* We do not need the return value.  */
> -  __execve (buf, argv, envp);
> +     mounted we fail.  We do not need the return value.  */
> +  struct fd_to_filename filename;
> +  __execve (__fd_to_filename (fd, &filename), argv, envp);
>  
>    int save = errno;
>  
> 

Re: [RFC v2] linux: use __fd_to_filename helper function instead of snprintf.

[Érico Nogueira]

On Mon May 3, 2021 at 4:26 PM -03, Adhemerval Zanella wrote:
>
>
> On 27/04/2021 10:09, Érico Nogueira via Libc-alpha wrote:
> > Change made to fchmodat and fexecve. There are tests using xasprintf
> > instead of this helper as well, but this commit doesn't touch them.
>
> To adapt the tests it would require either to build it as
> internal/static
> or add fd_to_filename to libsupport. I am not this is really an
> improvement.

That might explain some of the build failures I got when I tried
changing the tests.

>
> LGTM, thanks. I will commit this for you.

Thanks!

I found a few more places where __fd_to_filename could be used, they are
currently using _fitoa_word together with stpcpy. Would you be
interested in making __fd_to_filename use _fitoa_word or _itoa_word? Or
should it be left as is?

>
> Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
>
> > ---
> > 
> > Differences from v1:
> > - actually builds
> > - doesn't use an intermediary buf variable
> > 
> >  sysdeps/unix/sysv/linux/fchmodat.c | 13 +++----------
> >  sysdeps/unix/sysv/linux/fexecve.c  | 10 ++++------
> >  2 files changed, 7 insertions(+), 16 deletions(-)
> > 
> > diff --git a/sysdeps/unix/sysv/linux/fchmodat.c b/sysdeps/unix/sysv/linux/fchmodat.c
> > index f264f0c09d..5bd1eb96a5 100644
> > --- a/sysdeps/unix/sysv/linux/fchmodat.c
> > +++ b/sysdeps/unix/sysv/linux/fchmodat.c
> > @@ -18,6 +18,7 @@
> >  
> >  #include <errno.h>
> >  #include <fcntl.h>
> > +#include <fd_to_filename.h>
> >  #include <not-cancel.h>
> >  #include <stdio.h>
> >  #include <sys/stat.h>
> > @@ -69,16 +70,8 @@ fchmodat (int fd, const char *file, mode_t mode, int flag)
> >  
> >        /* For most file systems, fchmod does not operate on O_PATH
> >  	 descriptors, so go through /proc.  */
> > -      char buf[32];
> > -      if (__snprintf (buf, sizeof (buf), "/proc/self/fd/%d", pathfd) < 0)
> > -	{
> > -	  /* This also may report strange error codes to the caller
> > -	     (although snprintf really should not fail).  */
> > -	  __close_nocancel (pathfd);
> > -	  return -1;
> > -	}
> > -
> > -      int ret = __chmod (buf, mode);
> > +      struct fd_to_filename filename;
> > +      int ret = __chmod (__fd_to_filename (pathfd, &filename), mode);
> >        if (ret != 0)
> >  	{
> >  	  if (errno == ENOENT)
> > diff --git a/sysdeps/unix/sysv/linux/fexecve.c b/sysdeps/unix/sysv/linux/fexecve.c
> > index f37c245396..df25c2acb8 100644
> > --- a/sysdeps/unix/sysv/linux/fexecve.c
> > +++ b/sysdeps/unix/sysv/linux/fexecve.c
> > @@ -22,6 +22,7 @@
> >  #include <fcntl.h>
> >  #include <sys/stat.h>
> >  
> > +#include <fd_to_filename.h>
> >  #include <sysdep.h>
> >  #include <sys/syscall.h>
> >  #include <kernel-features.h>
> > @@ -49,12 +50,9 @@ fexecve (int fd, char *const argv[], char *const envp[])
> >  
> >  #ifndef __ASSUME_EXECVEAT
> >    /* We use the /proc filesystem to get the information.  If it is not
> > -     mounted we fail.  */
> > -  char buf[sizeof "/proc/self/fd/" + sizeof (int) * 3];
> > -  __snprintf (buf, sizeof (buf), "/proc/self/fd/%d", fd);
> > -
> > -  /* We do not need the return value.  */
> > -  __execve (buf, argv, envp);
> > +     mounted we fail.  We do not need the return value.  */
> > +  struct fd_to_filename filename;
> > +  __execve (__fd_to_filename (fd, &filename), argv, envp);
> >  
> >    int save = errno;
> >  
> > 

Re: [RFC v2] linux: use __fd_to_filename helper function instead of snprintf.

[Adhemerval Zanella]

On 03/05/2021 17:26, Érico Nogueira wrote:
> On Mon May 3, 2021 at 4:26 PM -03, Adhemerval Zanella wrote:
>>
>>
>> On 27/04/2021 10:09, Érico Nogueira via Libc-alpha wrote:
>>> Change made to fchmodat and fexecve. There are tests using xasprintf
>>> instead of this helper as well, but this commit doesn't touch them.
>>
>> To adapt the tests it would require either to build it as
>> internal/static
>> or add fd_to_filename to libsupport. I am not this is really an
>> improvement.
> 
> That might explain some of the build failures I got when I tried
> changing the tests.
> 
>>
>> LGTM, thanks. I will commit this for you.
> 
> Thanks!
> 
> I found a few more places where __fd_to_filename could be used, they are
> currently using _fitoa_word together with stpcpy. Would you be
> interested in making __fd_to_filename use _fitoa_word or _itoa_word? Or
> should it be left as is?

I think it might worth to check whether making __fd_to_filename use
_itoa_word yield any code size gain, but in general it seems a good
refactor.