Re: [f2fs-dev] [External Mail]Re: [PATCH v3] f2fs-tools: fix to check loop device for non-root users

[Jaegeuk Kim <jaegeuk@kernel.org>]

On 03/01, 黄佳男 wrote:
> On 2024/3/1 16:39, Juhyung Park wrote:
> > [外部邮件] 此邮件来源于小米公司外部，请谨慎处理。若对邮件安全性存疑，请将邮件转发给misec@xiaomi.com进行反馈
> >
> > Hi Huang and Chao.
> >
> > I feel like this special loopback handling alongside Chao's
> > 14197d546b93 on f2fs-tools is just unnecessarily complicating the code
> > flow.
> > We're now doing what, lookup to /sys, parse original backing file,
> > remove trailing newline char, stat()'ing it to make sure it exists?
> 
> Indeed this is not a good approach.
> 
> > What if the stat()'ed file is a new file after the original backing
> > file has been deleted?
> >
> > Being able to overwrite an active loopback backing file is a semantic
> > that Linux provides willingly.
> > O_EXCL only works on block devices and it's a POSIX guarantee that
> > multiple writers can work on a regular file.
> >
> > IMHO we should honor that, but if we really want to prevent this akin
> > to e2fsprogs, we should be using mntent like e2fsprogs.
> 
> e2fsprogs will continue to check if opening the loop device fails, 
> rather than
> 
> exiting. This way non-root users can use mkfs/fsck normally, although there
> 
> may be overwrite issues.

I think we need to fix this first and try to find a better way.

https://lore.kernel.org/linux-f2fs-devel/20240305204834.101697-1-jaegeuk@kernel.org/T/#u

> 
> 
> Thanks,
> 
> Jianan
> 
> > On Fri, Mar 1, 2024 at 4:15 PM Huang Jianan via Linux-f2fs-devel
> > <linux-f2fs-devel@lists.sourceforge.net> wrote:
> >> Currently mkfs/fsck gets the following error when executed by
> >> non-root users:
> >>
> >> Info: open /dev/loop0 failed errno:13
> >>          Error: Not available on mounted device!
> >>
> >> Let's fix it by reading the backing file from sysfs.
> >>
> >> Fixes: 14197d546b93 ("f2fs-tools: fix to check loop device")
> >> Signed-off-by: Huang Jianan <huangjianan@xiaomi.com>
> >> ---
> >> v3:
> >> - Skip deleted backing file.
> >>   lib/libf2fs.c | 40 +++++++++++++++++++++++++++-------------
> >>   1 file changed, 27 insertions(+), 13 deletions(-)
> >>
> >> diff --git a/lib/libf2fs.c b/lib/libf2fs.c
> >> index d51e485..fad3fd4 100644
> >> --- a/lib/libf2fs.c
> >> +++ b/lib/libf2fs.c
> >> @@ -832,7 +832,7 @@ int f2fs_dev_is_umounted(char *path)
> >>                  }
> >>          } else if (S_ISREG(st_buf.st_mode)) {
> >>                  /* check whether regular is backfile of loop device */
> >> -#if defined(HAVE_LINUX_LOOP_H) && defined(HAVE_LINUX_MAJOR_H)
> >> +#if defined(HAVE_LINUX_MAJOR_H)
> >>                  struct mntent *mnt;
> >>                  struct stat st_loop;
> >>                  FILE *f;
> >> @@ -840,8 +840,9 @@ int f2fs_dev_is_umounted(char *path)
> >>                  f = setmntent("/proc/mounts", "r");
> >>
> >>                  while ((mnt = getmntent(f)) != NULL) {
> >> -                       struct loop_info64 loopinfo = {0, };
> >> -                       int loop_fd, err;
> >> +                       struct stat st_back;
> >> +                       int sysfs_fd, rc;
> >> +                       char buf[PATH_MAX + 1];
> >>
> >>                          if (mnt->mnt_fsname[0] != '/')
> >>                                  continue;
> >> @@ -852,23 +853,36 @@ int f2fs_dev_is_umounted(char *path)
> >>                          if (major(st_loop.st_rdev) != LOOP_MAJOR)
> >>                                  continue;
> >>
> >> -                       loop_fd = open(mnt->mnt_fsname, O_RDONLY);
> >> -                       if (loop_fd < 0) {
> >> +                       snprintf(buf, PATH_MAX,
> >> +                                "/sys/dev/block/%d:%d/loop/backing_file",
> >> +                                major(st_loop.st_rdev), minor(st_loop.st_rdev));
> >> +
> >> +                       sysfs_fd = open(buf, O_RDONLY);
> >> +                       if (sysfs_fd < 0) {
> >>                                  MSG(0, "Info: open %s failed errno:%d\n",
> >> -                                       mnt->mnt_fsname, errno);
> >> +                                       buf, errno);
> >>                                  return -1;
> >>                          }
> >>
> >> -                       err = ioctl(loop_fd, LOOP_GET_STATUS64, &loopinfo);
> >> -                       close(loop_fd);
> >> -                       if (err < 0) {
> >> -                               MSG(0, "\tError: ioctl LOOP_GET_STATUS64 failed errno:%d!\n",
> >> -                                       errno);
> >> +                       memset(buf, 0, PATH_MAX + 1);
> >> +                       rc = read(sysfs_fd, buf, PATH_MAX);
> >> +                       if (rc < 0) {
> >> +                               MSG(0, "Info: read %s failed errno:%d\n",
> >> +                                       buf, errno);
> >>                                  return -1;
> >>                          }
> >>
> >> -                       if (st_buf.st_dev == loopinfo.lo_device &&
> >> -                               st_buf.st_ino == loopinfo.lo_inode) {
> >> +                       /* Remove trailing newline */
> >> +                       if (rc > 0 && *(buf + rc - 1) == '\n')
> >> +                               --rc;
> >> +                       buf[rc] = '\0';
> >> +
> >> +                       /* Skip deleted files like "xxx (deleted)" */
> >> +                       if (stat(buf, &st_back) != 0)
> >> +                               continue;
> >> +
> >> +                       if (st_buf.st_dev == st_back.st_dev &&
> >> +                               st_buf.st_ino == st_back.st_ino) {
> >>                                  MSG(0, "\tError: In use by loop device!\n");
> >>                                  return -EBUSY;
> >>                          }
> >> --
> >> 2.34.1
> >>
> >>
> >>
> >> _______________________________________________
> >> Linux-f2fs-devel mailing list
> >> Linux-f2fs-devel@lists.sourceforge.net
> >> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
> 
> 


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