[LTP] [PATCH] tst_find_backing_dev: fix logic in matching mount point

[Jan Stancek]

On Mon, Jul 11, 2022 at 3:21 PM Richard Palethorpe <rpalethorpe@suse.de> wrote:
>
> Hello,
>
> Jan Stancek <jstancek@redhat.com> writes:
>
> > On Mon, Jul 11, 2022 at 2:13 PM Richard Palethorpe <rpalethorpe@suse.de> wrote:
> >>
> >> Hello,
> >>
> >> Jan Stancek <jstancek@redhat.com> writes:
> >>
> >> > If backing dev is btrfs root device, then starting best_match_len
> >> > from 1 creates an issue, because root (/) is never matched.
> >> > Also we should check that entire mount point string is present in
> >> > path we are matching against.
> >> >
> >> > In case there's error also dump /proc/self/mountinfo before tst_brk.
> >> >
> >> > This fixes test with following partition layout (TMPDIR is on /):
> >> >   # cat /proc/self/mountinfo  | grep btrfs
> >> >   59 1 0:29 /root / rw,relatime shared:1 - btrfs /dev/dasda2 rw,seclabel,compress=zstd:1,ssd,space_cache=v2,subvolid=257,subvol=/root
> >> >   93 59 0:29 /home /home rw,relatime shared:47 - btrfs /dev/dasda2 rw,seclabel,compress=zstd:1,ssd,space_cache=v2,subvolid=256,subvol=/home
> >> >
> >> > Signed-off-by: Jan Stancek <jstancek@redhat.com>
> >> > ---
> >> >  lib/tst_device.c | 17 ++++++++++++-----
> >> >  1 file changed, 12 insertions(+), 5 deletions(-)
> >> >
> >> > diff --git a/lib/tst_device.c b/lib/tst_device.c
> >> > index c34cbe6d1f56..414bf0eea816 100644
> >> > --- a/lib/tst_device.c
> >> > +++ b/lib/tst_device.c
> >> > @@ -506,12 +506,17 @@ unsigned long tst_dev_bytes_written(const char *dev)
> >> >       return dev_bytes_written;
> >> >  }
> >> >
> >> > -static int count_match_len(const char *first, const char *second)
> >> > +static int str_starts_with(const char *str, const char *prefix)
> >> >  {
> >> >       int len = 0;
> >> >
> >> > -     while (*first && *first++ == *second++)
> >> > +     while (*prefix) {
> >> > +             if (!*str)
> >> > +                     return 0;
> >> > +             if (*str++ != *prefix++)
> >> > +                     return 0;
> >> >               len++;
> >> > +     }
> >>
> >> I'm not sure this is better than the original. It's a seperate cleanup
> >> in any case.
> >
> > The difference is that partial matches now returns 0. Previously it did not,
> > it only counted number of matching characters. So for example ("/foobar" "/foo")
> > appeared as better match than ("/foobar", "/")
>
> Ah, sorry.
>
> >
> >>
> >> >
> >> >       return len;
> >> >  }
> >> > @@ -524,7 +529,7 @@ void tst_find_backing_dev(const char *path, char *dev)
> >> >       char *pre = NULL;
> >> >       char *next = NULL;
> >> >       unsigned int dev_major, dev_minor, line_mjr, line_mnr;
> >> > -     unsigned int len, best_match_len = 1;
> >> > +     unsigned int len, best_match_len = 0;
> >> >       char mnt_point[PATH_MAX];
> >> >
> >> >       if (stat(path, &buf) < 0)
> >> > @@ -550,7 +555,7 @@ void tst_find_backing_dev(const char *path, char *dev)
> >> >                       break;
> >> >               }
> >> >
> >> > -             len = count_match_len(path, mnt_point);
> >> > +             len = str_starts_with(path, mnt_point);
> >> >               if (len > best_match_len) {
> >> >                       strcpy(dev, pre);
> >> >                       best_match_len = len;
> >> > @@ -559,8 +564,10 @@ void tst_find_backing_dev(const char *path, char *dev)
> >> >
> >> >       SAFE_FCLOSE(NULL, file);
> >> >
> >> > -     if (!*dev)
> >> > +     if (!*dev) {
> >> > +             tst_system("cat /proc/self/mountinfo");
> >> >               tst_brkm(TBROK, NULL, "Cannot find block device for %s", path);
> >> > +     }
> >> >
> >> >       if (stat(dev, &buf) < 0)
> >> >               tst_brkm(TWARN | TERRNO, NULL, "stat(%s) failed", dev);
> >> > --
> >> > 2.27.0
> >>
> >> Makes sense. However I suspect this function can be replaced with the
> >> standard library method used in tst_stat_mount_dev. I didn't try this
> >> before because I'm not sure why it scans mountinfo instead of mounts.
> >
> > /proc/self/mounts doesn't contain major/minor numbers, which is primarily what
> > tst_find_backing_dev is using.
>
> Does anyone know a situation where checking the mount path doesn't work?

You could mount multiple times to same mount path, which would make it harder
to find the right one.

> It seems like we are just checking the device numbers first for
> historical reasons and not because it is necessary.

Device numbers seem slightly less ambiguous.

>
> I haven't seen any failure reports for io_control01 which only checks
> the path with tst_stat_mount_dev. However this test only runs if you
> have CG v2 enabled by default.
>
> >
> >>
> >>
> >> --
> >> Thank you,
> >> Richard.
> >>
>
>
> --
> Thank you,
> Richard.
>