Re: [igt-dev] [PATCH i-g-t] lib/tests/igt_fork.c: Fix error in mmap() flags

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Oct 07, 2022 at 06:04:54PM +0300, Ville Syrjälä wrote:
> On Fri, Oct 07, 2022 at 09:52:41AM -0500, Jake Freeland wrote:
> > In subtest_leak(), mmap() is called with the flag PROT_WRITE,
> > but no PROT_READ. Later in the function, the mapped memory is
> > read using `children[i]`. In FreeBSD, the lack of PROT_READ
> > causes SIGSEGV. Adding the PROT_READ flag to the mmap() call
> > fixes this.
> 
> Isn't it a CPU architecture thing rather than an OS thing?
> Well, I guess it could be both in some cases.

So I was confused by this since x86 only has the present
bit and the r/w bit, and those can't express a write only
page.

The (always trustworthy) interwebs suggest that FreeBSD will
give an entirely unusable "mapping" when you ask for just
PROT_WRITE. Ie. a mapping that you cannot read or write.
Why? Who knows.

So based on that the fault should already happen when we
assign the pids to children[].

Anyways the patch looks correct so
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> 
> > 
> > Signed-off-by: Jake Freeland <jfree@freebsd.org>
> > ---
> >  lib/tests/igt_fork.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/lib/tests/igt_fork.c b/lib/tests/igt_fork.c
> > index d19d0945..d883aba4 100644
> > --- a/lib/tests/igt_fork.c
> > +++ b/lib/tests/igt_fork.c
> > @@ -109,7 +109,7 @@ __noreturn static void igt_fork_timeout_leak(void)
> >  __noreturn static void subtest_leak(void)
> >  {
> >  	pid_t *children =
> > -		mmap(0, 4096, PROT_WRITE, MAP_SHARED | MAP_ANON, -1, 0);
> > +		mmap(0, 4096, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANON, -1, 0);
> >  	const int num_children = 4096 / sizeof(*children);
> >  
> >  	igt_subtest_init(fake_argc, fake_argv);
> > -- 
> > 2.37.0 (Apple Git-136)
> 
> -- 
> Ville Syrjälä
> Intel

-- 
Ville Syrjälä
Intel