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    SubjectRe: [PATCH] check name_len before down_read xattr_sem and sb_read in ext2_xattr_get
    On Thu 22-07-10 08:03:18, shenghui wrote:
    > 2010/7/22 Jan Kara <jack@suse.cz>:
    > >> Hi,
    > >>        I walked through ext2_xattr_get, and felt that we can
    > >> do some optimization on it. For name_len check, it's done
    > >> after down xattr_sem and sb_read, both of which are time
    > >> consuming operation compared with strlen:
    > >>          down_read(&EXT2_I(inode)->xattr_sem);
    > >>  ...
    > >>          bh = sb_bread(inode->i_sb, EXT2_I(inode)->i_file_acl);
    > >>  ...
    > >>          /* find named attribute */
    > >>          name_len = strlen(name);
    > >>
    > >>          error = -ERANGE;
    > >>          if (name_len > 255)
    > >>                  goto cleanup;
    > >>
    > >>        Most of the case, you'll get one valid block, but if the
    > >> name len > 255, then the xattr_sem down and sb_bread operation
    > >> can be seen as a waste of time. So I think we'd better do
    > >> name len check as early as possible.
    > >>        Following is my patch, and it's against 2.6.35-rc4.
    > >> Please check it.
    > >>
    > >> Signed-off-by: Wang Sheng-Hui <crosslonelyover@gmail.com>
    > >> ---
    > >>  fs/ext2/xattr.c |   12 +++++++-----
    > >>  1 files changed, 7 insertions(+), 5 deletions(-)
    > >>
    > >> diff --git a/fs/ext2/xattr.c b/fs/ext2/xattr.c
    > >> index 7c39157..0b94d61 100644
    > >> --- a/fs/ext2/xattr.c
    > >> +++ b/fs/ext2/xattr.c
    > >> @@ -161,6 +161,13 @@ ext2_xattr_get(struct inode *inode, int name_index, const char *name,
    > >>
    > >>       if (name == NULL)
    > >>               return -EINVAL;
    > >> +
    > >> +     /* find named attribute */
    > >> +     name_len = strlen(name);
    > >> +     error = -ERANGE;
    > >> +     if (name_len > 255)
    > >> +             goto cleanup;
    > >  But you cannot go to cleanup here because you don't hold xattr_sem...
    > >
    >
    > Sorry, I'm a little confused by your words.
    > The patch just checks name_len, and it
    > doesn't need xattr_sem.
    Checking of name_len is fine as you did it. But I wanted to point out
    that if name_len is greater than 255, you then go to 'cleanup' label which
    tries to do up_read(&EXT2_I(inode)->xattr_sem). But that's a bug because
    after you moved the code, we don't hold xattr_sem at the moment we check
    name_len.

    Honza
    --
    Jan Kara <jack@suse.cz>
    SUSE Labs, CR
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