Re: [PATCH v2 2/3] depmod: prevent module dependency files corruption due to parallel invocation.

[Yauheni Kaliuta <yauheni.kaliuta@redhat.com>]

Hi, Michal!

>>>>> On Mon, 17 Dec 2018 23:07:43 +0100, Michal Suchánek  wrote:

 > On Mon, 17 Dec 2018 10:10:37 -0800
 > Lucas De Marchi <lucas.de.marchi@gmail.com> wrote:

 >> On Mon, Dec 10, 2018 at 2:03 PM Michal Suchanek <msuchanek@suse.de> wrote:
 >> >
 >> > Depmod does not use unique filename for temporary files. There is no
 >> > guarantee the user does not attempt to run mutiple depmod processes in
 >> > parallel. If that happens a temporary file might be created by
 >> > depmod(1st), truncated by depmod(2nd), and renamed to final name by
 >> > depmod(1st) resulting in corrupted file seen by user.
 >> >
 >> > Due to missing mkstempat() this is more complex than it should be.
 >> > Adding PID and timestamp to the filename should be reasonably reliable.
 >> > Adding O_EXCL as mkstemp does fails creating the file rather than
 >> > corrupting existing file.
 >> >
 >> > Signed-off-by: Michal Suchanek <msuchanek@suse.de>
 >> > ---
 >> >  tools/depmod.c | 12 +++++++++---
 >> >  1 file changed, 9 insertions(+), 3 deletions(-)
 >> >
 >> > diff --git a/tools/depmod.c b/tools/depmod.c
 >> > index 18c0d61b2db3..3b6d16e76160 100644
 >> > --- a/tools/depmod.c
 >> > +++ b/tools/depmod.c
 >> > @@ -29,6 +29,7 @@
 >> >  #include <string.h>
 >> >  #include <unistd.h>
 >> >  #include <sys/stat.h>
 >> > +#include <sys/time.h>
 >> >  #include <sys/utsname.h>
 >> >
 >> >  #include <shared/array.h>
 >> > @@ -2398,6 +2399,9 @@ static int depmod_output(struct depmod *depmod, FILE *out)
 >> >         };
 >> >         const char *dname = depmod->cfg->dirname;
 >> >         int dfd, err = 0;
 >> > +       struct timeval tv;
 >> > +
 >> > +       gettimeofday(&tv, NULL);
 >> >
 >> >         if (out != NULL)
 >> >                 dfd = -1;
 >> > @@ -2412,15 +2416,17 @@ static int depmod_output(struct depmod *depmod, FILE *out)
 >> >
 >> >         for (itr = depfiles; itr->name != NULL; itr++) {
 >> >                 FILE *fp = out;
 >> > -               char tmp[NAME_MAX] = "";
 >> > +               char tmp[NAME_MAX + 1] = "";
 >> >                 int r, ferr;
 >> >
 >> >                 if (fp == NULL) {
 >> > -                       int flags = O_CREAT | O_TRUNC | O_WRONLY;
 >> > +                       int flags = O_CREAT | O_TRUNC | O_WRONLY | O_EXCL;
 >> >                         int mode = 0644;
 >> >                         int fd;
 >> >
 >> > -                       snprintf(tmp, sizeof(tmp), "%s.tmp", itr->name);
 >> > +                       snprintf(tmp, sizeof(tmp), "%s.%i.%li.%li", itr->name, getpid(),
 >> > +                                       tv.tv_usec, tv.tv_sec);
 >> > +                       tmp[NAME_MAX] = 0;  
 >> 
 >> why? snprintf is guaranteed to nul-terminate the string.
 >> 
 > AFAIK it is guaranteed to not write after end of buffer. It is
 > not guaranteed to terminate the string. To guarantee
 > terminated string you need large enough buffer or a nul after
 > the buffer. Or asprintf.

Actually, it is. VS strncpy(). May be it is not so clear from the
man page:

```
The functions snprintf() and vsnprintf() write at most size
bytes (including the terminating null byte ('\0')) to str.
```

-- 
WBR,
Yauheni Kaliuta