Messages in this thread | | | Date | Thu, 23 Apr 2020 04:12:53 +0300 | From | "Dmitry V. Levin" <> | Subject | Re: [PATCH v5 1/3] fs: Support setting a minimum fd for "lowest available fd" allocation |
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On Tue, Apr 21, 2020 at 10:19:49PM -0700, Josh Triplett wrote: > Some applications want to prevent the usual "lowest available fd" > allocation from allocating certain file descriptors. For instance, they > may want to prevent allocation of a closed fd 0, 1, or 2 other than via > dup2/dup3, or reserve some low file descriptors for other purposes. > > Add a prctl to increase the minimum fd and return the previous minimum. > > System calls that allocate a specific file descriptor, such as > dup2/dup3, ignore this minimum. > > exec resets the minimum fd, to prevent one program from interfering with > another program's expectations about fd allocation.
Please make this aspect properly documented in "Effect on process attributes" section of execve(2) manual page.
[...] > +unsigned int increase_min_fd(unsigned int num) > +{ > + struct files_struct *files = current->files; > + unsigned int old_min_fd; > + > + spin_lock(&files->file_lock); > + old_min_fd = files->min_fd; > + files->min_fd += num; > + spin_unlock(&files->file_lock); > + return old_min_fd; > +}
If it's "increase", there should be an overflow check. Otherwise it's "assign" rather than "increase".
-- ldv
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