Re: readdir(3): behavior on descriptors with O_SEARCH

[Dmitry Selyutin <ghostman.sd@gmail.com>]

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Hi Markus,

thank you for your reply!

> POSIX doesn't know O_SEARCH or O_PATH, and thus mandates nothing about
> their meaning.
POSIX 2008 with 2013 corrigenda mentions both O_SEARCH and O_EXEC.
http://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html

> Try strace(1). That should tell you what glibc and musl are doing
> differently. For instace, whether glibc removes O_SEARCH from the fd.
Thank you! I'll try it and report the results here.

FWIW it does not seem to be correct that O_SEARCH can be equal to O_PATH;
from what I gathered from manuals and various mailing lists discussions,
O_SEARCH may be equal to O_EXEC since each of these flags is valid either
for directory or file respectively.
28 авг. 2016 г. 10:11 пользователь "Markus Wichmann" <nullplan@gmx.net>
написал:

> On Sat, Aug 27, 2016 at 09:23:50PM +0300, Dmitry Selyutin wrote:
> > int const flags = (O_DIRECTORY | O_SEARCH);
> > int descriptor = open(path, flags);
> > DIR *handle = fdopendir(descriptor);
> > struct dirent *entry = readdir(handle);
> >
> > To cut the long story short, any attempt to call readdir(3) on directory
> > handle obtained via fdopendir(3) returns NULL and sets the errno variable
> > to EBADF. This behavior arises only on descriptors opened with
> (O_DIRECTORY
> > | O_SEARCH) flags enabled; it goes away if O_SEARCH flag is removed.
> >
>
> Try strace(1). That should tell you what glibc and musl are doing
> differently. For instace, whether glibc removes O_SEARCH from the fd.
>
> musl defines O_SEARCH to be equal to O_PATH. The manpage says that
> O_PATH means the file isn't opened for reading. I guess if you do that
> then getdents(2) will fail, which is what musl uses to implement
> readdir(3).
>
> I tried to do the same trace in glibc 2.19 (which is what Debian stable
> is using right now), but to no avail: O_SEARCH isn't even mentioned
> anywhere in that code. But its implementation of fdopendir(3) rejects
> fds open only for writing. The readdir(3) implementation is, of course,
> overcomplicated, but also seems to just call getdents(2). And then it
> tries to pack the kernel structures into its own structures, probably
> for ABI reasons. And people wonder why I dislike dynamic linking...
>
> > So it seems that O_SEARCH is the reason; I thought that this flag tells
> > exactly "well, I'm going to use it for search only", which implies "well,
> > I'm going to use only readdir(3) to get information about files inside".
> Is
> > my interpretation correct?
> >
>
> My manpage doesn't know O_SEARCH, but it knows O_PATH, and then you're
> wrong. It means "I'll only use this fd in *at() and fchdir() and
> similar; this fd isn't open for reading."
>
> > I'm not really sure if it is a bug, since I suspect POSIX may allow
> open(3)
> > with (O_DIRECTORY | O_SEARCH) flags to behave in an
> implementation-defined
> > matter; it can be possible that file descriptors obtained via open(3)
> with
> > O_DIRECTORY flag set are guaranteed to work only with fchdir(3) and
> *at(3)
> > operations. However, if such behavior is intentional, it would be a good
> > idea (in my opinion) make fdopendir(3) return NULL (though it won't match
> > behavior e.g. for glibc).
> >
>
> POSIX doesn't know O_SEARCH or O_PATH, and thus mandates nothing about
> their meaning.
>
> Ciao,
> Markus
>

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