On Sun, Mar 01, 2020 at 02:54:11AM +1100, Aleksa Sarai wrote:

On 2020-03-01, Aleksa Sarai [off-list ref] wrote:

quoted

On 2020-02-28, Christian Brauner [off-list ref] wrote:

quoted

So we either end up adding new AT_* flags mirroring the new RESOLVE_*
flags or we end up adding new RESOLVE_* flags mirroring parts of AT_*
flags. And if that's a possibility I vote for RESOLVE_* flags going
forward. The have better naming too imho.

I can see the argument for merging AT_ flags into RESOLVE_ flags (fewer
flag arguments for syscalls is usually a good thing) ... but I don't
really like it. There are a couple of problems right off the bat:

 * The prefix RESOLVE_ implies that the flag is specifically about path
   resolution. While you could argue that AT_EMPTY_PATH is at least
   *related* to path resolution, flags like AT_REMOVEDIR and
   AT_RECURSIVE aren't.

 * That point touches on something I see as a more fundamental problem
   in the AT_ flags -- they were intended to be generic flags for all of
   the ...at(2) syscalls. But then AT_ grew things like AT_STATX_ and
   AT_REMOVEDIR (both of which are necessary features to have for their
   respective syscalls, but now those flag bits are dead for other
   syscalls -- not to mention the whole AT_SYMLINK_{NO,}FOLLOW thing).

 * While the above might be seen as minor quibbles, the really big
   issue is that even the flags which are "similar" (AT_SYMLINK_NOFOLLOW
   and RESOLVE_NO_SYMLINKS) have different semantics (by design -- in my
   view, AT_SYMLINK_{NO,}FOLLOW / O_NOFOLLOW / lstat(2) has always had
   the wrong semantics if the intention was to be a way to safely avoid
   resolving symlinks).

But maybe I'm just overthinking what a merge of AT_ and RESOLVE_ would
look like -- would it on.

Eugh, dropped the rest of that sentence:

... would it only be the few AT_ flags which are strictly related to
path resolution (such as AT_EMPTY_PATH)? If so wouldn't that just mean
we end up with two flag arguments for new syscalls?

That's a good question that we kinda ran into right once we
accepted the RESOLVE_* namespace implicitly? This smells like the same
problem we have in e.g. waitid() with WEXITED/WSTOPPED/WCONTINUED and
WNOHANG/WNOWAIT...I think one answer could be one flag argument,
different prefixes? i.e. RESOLVE_* and then e.g. simply REMOVE_DIR instead of
AT_REMOVEDIR. This way we don't duplicate the problem the AT_*
namespace had (e.g. AT_REMOVEDIR and AT_SYMLINK_NOFOLLOW being about two
separate things). Maybe that's crazy and doesn't really make things
better?

Christian