[PATCH mdadm v2 04/14] mdadm/Grow: Fix use after close bug by closing after fork

[Logan Gunthorpe <logang@deltatee.com>]

The test 07reshape-grow fails most of the time. But it succeeds around
1 in 5 times. When it does succeed, it causes the tests to die because
mdadm has segfaulted.

The segfault was caused by mdadm attempting to repoen a file
descriptor that was already closed. The backtrace of the segfault
was:

  #0  __strncmp_avx2 () at ../sysdeps/x86_64/multiarch/strcmp-avx2.S:101
  #1  0x000056146e31d44b in devnm2devid (devnm=0x0) at util.c:956
  #2  0x000056146e31dab4 in open_dev_flags (devnm=0x0, flags=0)
                         at util.c:1072
  #3  0x000056146e31db22 in open_dev (devnm=0x0) at util.c:1079
  #4  0x000056146e3202e8 in reopen_mddev (mdfd=4) at util.c:2244
  #5  0x000056146e329f36 in start_array (mdfd=4,
              mddev=0x7ffc55342450 "/dev/md0", content=0x7ffc55342860,
              st=0x56146fc78660, ident=0x7ffc55342f70, best=0x56146fc6f5d0,
              bestcnt=10, chosen_drive=0, devices=0x56146fc706b0, okcnt=5,
	      sparecnt=0,  rebuilding_cnt=0, journalcnt=0, c=0x7ffc55342e90,
	      clean=1,  avail=0x56146fc78720 "\001\001\001\001\001",
	      start_partial_ok=0, err_ok=0, was_forced=0)
	                  at Assemble.c:1206
  #6  0x000056146e32c36e in Assemble (st=0x56146fc78660,
               mddev=0x7ffc55342450 "/dev/md0", ident=0x7ffc55342f70,
	       devlist=0x56146fc6e2d0, c=0x7ffc55342e90)
	                 at Assemble.c:1914
  #7  0x000056146e312ac9 in main (argc=11, argv=0x7ffc55343238)
                         at mdadm.c:1510

The file descriptor was closed early in Grow_continue(). The noted commit
moved the close() call to close the fd above the fork which caused the
parent process to return with a closed fd.

This meant reshape_array() and Grow_continue() would return in the parent
with the fd forked. The fd would eventually be passed to reopen_mddev()
which returned an unhandled NULL from fd2devnm() which would then be
dereferenced in devnm2devid.

Fix this by moving the close() call below the fork. This appears to
fix the 07revert-grow test. While we're at it, switch to using
close_fd() to invalidate the file descriptor.

Fixes: 77b72fa82813 ("mdadm/Grow: prevent md's fd from being occupied during delayed time")
Cc: Alex Wu <alexwu@synology.com>
Cc: BingJing Chang <bingjingc@synology.com>
Cc: Danny Shih <dannyshih@synology.com>
Cc: ChangSyun Peng <allenpeng@synology.com>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
---
 Grow.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/Grow.c b/Grow.c
index f6efbc48dafd..0e2d7181bcab 100644
--- a/Grow.c
+++ b/Grow.c
@@ -3514,7 +3514,6 @@ started:
 			return 0;
 		}
 
-	close(fd);
 	/* Now we just need to kick off the reshape and watch, while
 	 * handling backups of the data...
 	 * This is all done by a forked background process.
@@ -3535,6 +3534,9 @@ started:
 		break;
 	}
 
+	/* Close unused file descriptor in the forked process */
+	close_fd(&fd);
+
 	/* If another array on the same devices is busy, the
 	 * reshape will wait for them.  This would mean that
 	 * the first section that we suspend will stay suspended
-- 
2.30.2