From: Al Viro Date: Fri, 9 May 2008 01:19:42 +0000 (-0400) Subject: [PATCH] dup_fd() part 4 - race fix X-Git-Tag: v2.6.26-rc4~115^2~5 X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=adbecb128cd2cc5d14b0ebef6d020ced0efd0ec6;p=linux-2.6 [PATCH] dup_fd() part 4 - race fix Parent _can_ be a clone task, contrary to the comment. Moreover, more files could be opened while we allocate a copy, in which case we end up copying only part into new descriptor table. Since what we get _is_ affected by all changes in the old range, we can get rather weird effects - e.g. dup2(0, 1024); close(0); in parallel with fork() resulting in child that sees the effect of close(), but not that of dup2() done just before that close(). What we need is to recalculate the open_count after having reacquired ->file_lock and if external fdtable we'd just allocated is too small for it, free the sucker and redo allocation. Signed-off-by: Al Viro --- diff --git a/fs/file.c b/fs/file.c index 689d2b6947..0f705c7cfe 100644 --- a/fs/file.c +++ b/fs/file.c @@ -308,11 +308,16 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp) /* * Check whether we need to allocate a larger fd array and fd set. - * Note: we're not a clone task, so the open count won't change. */ - if (open_files > new_fdt->max_fds) { + while (unlikely(open_files > new_fdt->max_fds)) { spin_unlock(&oldf->file_lock); + if (new_fdt != &newf->fdtab) { + free_fdarr(new_fdt); + free_fdset(new_fdt); + kfree(new_fdt); + } + new_fdt = alloc_fdtable(open_files - 1); if (!new_fdt) { *errorp = -ENOMEM; @@ -335,6 +340,7 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp) */ spin_lock(&oldf->file_lock); old_fdt = files_fdtable(oldf); + open_files = count_open_files(old_fdt); } old_fds = old_fdt->fd;