fat: Add allow_utime option

Normally utime(2) checks current process is owner of the file, or it
has CAP_FOWNER capability.  But FAT filesystem doesn't have uid/gid as
on disk info, so normal check is too unflexible.

With this option you can relax it.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 55 insertions, 3 deletions

diff --git a/Documentation/filesystems/vfat.txt b/Documentation/filesystems/vfat.txt
index fcc123ffa252..2d5e1e582e13 100644
--- a/Documentation/filesystems/vfat.txt
+++ b/Documentation/filesystems/vfat.txt
@@ -17,6 +17,21 @@ dmask=###     -- The permission mask for the directory.
 fmask=###     -- The permission mask for files.
                  The default is the umask of current process.
 
+allow_utime=### -- This option controls the permission check of mtime/atime.
+
+                  20 - If current process is in group of file's group ID,
+                       you can change timestamp.
+                   2 - Other users can change timestamp.
+
+                 The default is set from `dmask' option. (If the directory is
+                 writable, utime(2) is also allowed. I.e. ~dmask & 022)
+
+                 Normally utime(2) checks current process is owner of
+                 the file, or it has CAP_FOWNER capability.  But FAT
+                 filesystem doesn't have uid/gid on disk, so normal
+                 check is too unflexible. With this option you can
+                 relax it.
+
 codepage=###  -- Sets the codepage number for converting to shortname
                  characters on FAT filesystem.
                  By default, FAT_DEFAULT_CODEPAGE setting is used.
diff --git a/fs/fat/file.c b/fs/fat/file.c
index e73f13a13792..d604bb132422 100644
--- a/fs/fat/file.c
+++ b/fs/fat/file.c
@@ -280,11 +280,27 @@ static int fat_check_mode(const struct msdos_sb_info *sbi, struct inode *inode,
         return 0;
 }
 
+static int fat_allow_set_time(struct msdos_sb_info *sbi, struct inode *inode)
+{
+        mode_t allow_utime = sbi->options.allow_utime;
+
+        if (current->fsuid != inode->i_uid) {
+                if (in_group_p(inode->i_gid))
+                        allow_utime >>= 3;
+                if (allow_utime & MAY_WRITE)
+                        return 1;
+        }
+
+        /* use a default check */
+        return 0;
+}
+
 int fat_setattr(struct dentry *dentry, struct iattr *attr)
 {
         struct msdos_sb_info *sbi = MSDOS_SB(dentry->d_sb);
         struct inode *inode = dentry->d_inode;
         int mask, error = 0;
+        unsigned int ia_valid;
 
         lock_kernel();
 
@@ -302,7 +318,15 @@ int fat_setattr(struct dentry *dentry, struct iattr *attr)
                 }
         }
 
+        /* Check for setting the inode time. */
+        ia_valid = attr->ia_valid;
+        if (ia_valid & (ATTR_MTIME_SET | ATTR_ATIME_SET)) {
+                if (fat_allow_set_time(sbi, inode))
+                        attr->ia_valid &= ~(ATTR_MTIME_SET | ATTR_ATIME_SET);
+        }
+
         error = inode_change_ok(inode, attr);
+        attr->ia_valid = ia_valid;
         if (error) {
                 if (sbi->options.quiet)
                         error = 0;
diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index f22f287e507a..886f6095a378 100644
--- a/fs/fat/inode.c
+++ b/fs/fat/inode.c
@@ -785,6 +785,8 @@ static int fat_show_options(struct seq_file *m, struct vfsmount *mnt)
                 seq_printf(m, ",gid=%u", opts->fs_gid);
         seq_printf(m, ",fmask=%04o", opts->fs_fmask);
         seq_printf(m, ",dmask=%04o", opts->fs_dmask);
+        if (opts->allow_utime)
+                seq_printf(m, ",allow_utime=%04o", opts->allow_utime);
         if (sbi->nls_disk)
                 seq_printf(m, ",codepage=%s", sbi->nls_disk->charset);
         if (isvfat) {
@@ -840,9 +842,9 @@ static int fat_show_options(struct seq_file *m, struct vfsmount *mnt)
 
 enum {
         Opt_check_n, Opt_check_r, Opt_check_s, Opt_uid, Opt_gid,
-        Opt_umask, Opt_dmask, Opt_fmask, Opt_codepage, Opt_usefree, Opt_nocase,
-        Opt_quiet, Opt_showexec, Opt_debug, Opt_immutable,
-        Opt_dots, Opt_nodots,
+        Opt_umask, Opt_dmask, Opt_fmask, Opt_allow_utime, Opt_codepage,
+        Opt_usefree, Opt_nocase, Opt_quiet, Opt_showexec, Opt_debug,
+        Opt_immutable, Opt_dots, Opt_nodots,
         Opt_charset, Opt_shortname_lower, Opt_shortname_win95,
         Opt_shortname_winnt, Opt_shortname_mixed, Opt_utf8_no, Opt_utf8_yes,
         Opt_uni_xl_no, Opt_uni_xl_yes, Opt_nonumtail_no, Opt_nonumtail_yes,
@@ -861,6 +863,7 @@ static match_table_t fat_tokens = {
         {Opt_umask, "umask=%o"},
         {Opt_dmask, "dmask=%o"},
         {Opt_fmask, "fmask=%o"},
+        {Opt_allow_utime, "allow_utime=%o"},
         {Opt_codepage, "codepage=%u"},
         {Opt_usefree, "usefree"},
         {Opt_nocase, "nocase"},
@@ -932,6 +935,7 @@ static int parse_options(char *options, int is_vfat, int silent, int *debug,
         opts->fs_uid = current->uid;
         opts->fs_gid = current->gid;
         opts->fs_fmask = opts->fs_dmask = current->fs->umask;
+        opts->allow_utime = -1;
         opts->codepage = fat_default_codepage;
         opts->iocharset = fat_default_iocharset;
         if (is_vfat)
@@ -1019,6 +1023,11 @@ static int parse_options(char *options, int is_vfat, int silent, int *debug,
                                 return 0;
                         opts->fs_fmask = option;
                         break;
+                case Opt_allow_utime:
+                        if (match_octal(&args[0], &option))
+                                return 0;
+                        opts->allow_utime = option & (S_IWGRP | S_IWOTH);
+                        break;
                 case Opt_codepage:
                         if (match_int(&args[0], &option))
                                 return 0;
@@ -1101,6 +1110,9 @@ static int parse_options(char *options, int is_vfat, int silent, int *debug,
                        " for FAT filesystems, filesystem will be case sensitive!\n");
         }
 
+        /* If user doesn't specify allow_utime, it's initialized from dmask. */
+        if (opts->allow_utime == (unsigned short)-1)
+                opts->allow_utime = ~opts->fs_dmask & (S_IWGRP | S_IWOTH);
         if (opts->unicode_xlate)
                 opts->utf8 = 0;
 
diff --git a/include/linux/msdos_fs.h b/include/linux/msdos_fs.h
index c4bed98bc00b..81fb9bc53220 100644
--- a/include/linux/msdos_fs.h
+++ b/include/linux/msdos_fs.h
@@ -195,6 +195,7 @@ struct fat_mount_options {
         char *iocharset;          /* Charset used for filename input/display */
         unsigned short shortname; /* flags for shortname display/create rule */
         unsigned char name_check; /* r = relaxed, n = normal, s = strict */
+        unsigned short allow_utime;/* permission for setting the [am]time */
         unsigned quiet:1,         /* set = fake successful chmods and chowns */
                  showexec:1,      /* set = only set x bit for com/exe/bat */
                  sys_immutable:1, /* set = system files are immutable */