Re: make PROT_WRITE imply PROT_READ

[Jason Baron <jbaron@redhat.com>]

On Thu, 22 Jun 2006, Robert Hancock wrote:

> Jason Baron wrote:
> > Currently, if i mmap() a file PROT_WRITE only and then first read from it
> > and then write to it, i get a SEGV. However, if i write to it first and then
> > read from it, i get no SEGV. This seems rather inconsistent.
> > 
> > The current implementation seems to be to make PROT_WRITE imply PROT_READ,
> > however it does not quite work correctly. The patch below resolves this
> > issue, by explicitly setting the PROT_READ flag when PROT_WRITE is
> > requested.
> 
> I would disagree.. the kernel is enforcing the permissions specified where the
> CPU architecture allows it. There is no sense in breaking this everywhere just
> because we can't always enforce it. By that logic we should be making
> PROT_READ imply PROT_EXEC because not all CPUs can enforce them separately,
> which makes no sense at all.
> 
> > 
> > This might appear at first as a possible permissions subversion, as i could
> > get PROT_READ on a file that i only have write permission to...however, the
> > mmap implementation requires that the file be opened with at least read
> > access already. Thus, i don't believe there is any issue with regards to
> > permissions.
> > 
> > Another consequenece of this patch is that it forces PROT_READ even for
> > architectures that might be able to support it, (I know that x86, x86_64 and
> > ia64 do not) but i think this is best for portability.
> 
> That makes little sense to me.. if you want portability, and you're reading
> from the file, you better request PROT_READ. Any app that doesn't do that is
> inherently broken and non-portable regardless of what you do to the kernel.
> 
> 


hi,

So if i create a PROT_WRITE only mapping and then read from first and then 
writte to it a get a SEGV. However, if i write to it first and then read 
from it, i don't get a SEGV...Why should the read/write ordering matter? 

thanks,

-Jason