[PATCH man-pages] open.2: improve O_PATH documentation.

[NeilBrown <neilb@suse.com>]

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- fstatfs is now permitted.
- ioctl isn't, and is worth listing explicitly
- O_PATH allows an automount point to be opened with
  triggering the mount.

All tested

Signed-off-by: NeilBrown <neilb@suse.com>
---
 man2/open.2 | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/man2/open.2 b/man2/open.2
index e3b00939790c..38d532397b02 100644
--- a/man2/open.2
+++ b/man2/open.2
@@ -585,6 +585,7 @@ The file itself is not opened, and other file operations (e.g.,
 .BR fchmod (2),
 .BR fchown (2),
 .BR fgetxattr (2),
+.BR ioctl (2),
 .BR mmap (2))
 fail with the error
 .BR EBADF .
@@ -599,8 +600,11 @@ be performed on the resulting file descriptor:
 (since Linux 3.5);
 .\" commit 332a2e1244bd08b9e3ecd378028513396a004a24
 .BR fstat (2)
-(since Linux 3.6).
+(since Linux 3.6);
 .\" fstat(): commit 55815f70147dcfa3ead5738fd56d3574e2e3c1c2
+.BR fstatfs (2)
+(since Linux 3.12).
+.\" fstatfs(): commit 9d05746e7b16d8565dddbe3200faa1e669d23bbf
 .IP *
 Duplicating the file descriptor
 .RB ( dup (2),
@@ -666,6 +670,16 @@ argument in calls to
 and
 .BR readlinkat (2)
 with an empty pathname to have the calls operate on the symbolic link.
+
+If
+.I pathname
+refers to an automount point that has not yet been triggered, so no
+other filesystem is mounted on it, then the call returns a file
+descriptor referring to the automount directory without triggering a mount.
+.BR fstatfs (2)
+can then be used to determine if it is, in fact, an untriggered
+automount point
+.RB ( ".f_type == AUTOFS_SUPER_MAGIC" ).
 .TP
 .B O_SYNC
 Write operations on the file will complete according to the requirements of
-- 
2.14.0.rc0.dirty


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