On Tue, Aug 27, 2013 at 12:32 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Aug 27, 2013 at 12:16 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> This is an experiment to see if we can get nice semantics for all syscalls
>> that either follow symlinks or allow AT_EMPTY_PATH without jumping through
>> enormous hoops. This converts truncate (although you can't tell using
>> truncate from coreutils, because it actually uses open + ftruncate).
>
> So this seems *way* too complex. I'd much rather see "nd->flags" get a
> LOOKUP_READONLY flag, for example, that gets set by
> proc_pid_follow_link() when it hits a read-only file descriptor (and
> gets cleared by other lookups).
>
> Wouldn't that be *much* more straightforward?
It would if it works. It certainly would for truncate, setxattr, etc.
There are funny cases, though. For example, execing an O_WRONLY fd
should probably fail, as should opening an O_WRONLY fd as O_RDWR.
O_APPEND is also funny. flink will (I suspect) always want to be a
bit special.
There are also O_PATH fds, and I'm not sure what the semantics of
O_PATH fds are or should be when they refer to something other than a
directory.
The benefit of my approach is that it's really obvious that
truncate("/proc/self/fd/N") and ftruncate(N) do exactly the same
thing. The downside is that the namei code is a bit gross and there
was more rearranging of ftruncate than I would have liked. (I also
lost the benefit of fget_light, but I could fix that by passing a
struct fd around instead of a struct file *.)
--Andy
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