quoted

My biggest motivation was actually seeing xfs_io crashing due a sigbus
while running generic/172 and generic/173. And personally, I'd rather see an
error message like "attempt to mmap/mwrite beyond EOF" than seeing it crash.
Also, as you mentioned, programs are allowed to set up such kind of
configuration (IIUC what you mean, mixing mmap, extend, truncate, etc), so, I
believe such userspace programs should also ensure they are not attempting to
write to invalid memory.

This patch would /also/ prevent us from writing an fstest to check that
a process /does/ get SIGBUS when writing to a mapping beyond EOF.  Huh,
we don't have a test for that...

TBH, this kind of test in xfstests didn't pass through my mind, but, this is
mmap (and its mapped regions) behavior, I don't see why we would want to test
this in xfstests, but, well, I am neither a memory expert to say what it
should/shouldn't do (I'm just following the man page), nor I have any authority
to say what we can/can't do in xfstests :)

Also, where does generic/173 write to a mapping beyond EOF?  It sets up
a file of blksz*nr_blks bytes, clones it, fills the fs to full, and then
writes that number of bytes to the mmap region to trigger SIGBUS when
the COW fails due to ENOSPC.

The whole command that ends up receiving a SIGBUS is:

xfs_io -i -f -c 'mmap -rw 0 41943040' -c 'mwrite -S 0x62 0 41943040'

I have a slightly suspicious this is happening because it's trying to access the
very last byte of the file, but I do need to check again the machine where I was
running g/173. At a later point, I just did some tests using an empty, 0
sized file. So I do not recall from the top of my head the behavior from g/173.
I can do it later, now I need to run to a doc appt.

--D

quoted

quoted

OTOH if your goal is to write a test to check the SIGBUS functionality,
you could install a sigbus handler to report the signal to stderr, which
would avoid bash writing junk about the sigbus to the terminal.

No, I'm just trying to avoid xfs_io crashing if we point it to invalid memory :)

Cheers.

quoted

--D

quoted

 io/mmap.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/io/mmap.c b/io/mmap.c
index 9816cf68..77c5f2b6 100644
--- a/io/mmap.c
+++ b/io/mmap.c

@@ -242,6 +242,13 @@ mmap_f(
 		return 0;
 	}
 
+	/* Check if we are mmapping beyond EOF */
+	if ((offset + length) > filesize()) {
+		printf(_("Attempting to mmap() beyond EOF\n"));
+		exitcode = 1;
+		return 0;
+	}
+
 	/*
 	 * mmap and munmap memory area of length2 region is helpful to
 	 * make a region of extendible free memory. It's generally used

-- 
2.31.1

-- 
Carlos

-- 
Carlos