Re: [PATCH] Prevent mmap command to map beyond EOF

[Carlos Maiolino <cmaiolino@redhat.com>]

On Tue, Oct 05, 2021 at 03:36:53PM -0700, Darrick J. Wong wrote:
> On Mon, Oct 04, 2021 at 04:11:40PM +0200, Carlos Maiolino wrote:
> > Attempting to access a mmapp'ed region that does not correspond to the
> > file results in a SIGBUS, so prevent xfs_io to even attempt to mmap() a
> > region beyond EOF.
> > 
> > Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
> > ---
> > 
> > There is a caveat about this patch though. It is possible to mmap() a
> > non-existent file region, extent the file to go beyond such region, and run
> > operations in this mmapped region without such operations triggering a SIGBUS
> > (excluding the file corruption factor here :). So, I'm not quite sure if it
> > would be ok to check for this in mmap_f() as this patch does, or create a helper
> > to check for such condition, and use it on the other operations (mread_f,
> > mwrite_f, etc). What you folks think?
> 
> What's the motivation for checking this in userspace?  Programs are
> allowed to set up this (admittedly minimally functional) configuration,
> or even set it up after the mmap by truncating the file.

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.

> 
> 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.

> 
> --D
> 
> > 
> >  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