Messages in this thread | | | From | Linus Torvalds <> | Date | Wed, 5 Jul 2017 10:15:56 -0700 | Subject | Re: [PATCH] mm: larger stack guard gap, between vmas |
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On Wed, Jul 5, 2017 at 9:58 AM, Ben Hutchings <ben@decadent.org.uk> wrote: > > I ended up with the following two patches, which seem to deal with > both the Java and Rust regressions. These don't touch the > stack-grows-up paths at all because Rust doesn't run on those > architectures and the Java weirdness is i386-specific. > > They definitely need longer commit messages and comments, but aside > from that do these look reasonable?
I thin kthey both look reasonable, but I think we might still want to massage things a bit (cutting down the quoting to a minimum, hopefully leaving enough context to still make sense):
> Subject: [1/2] mmap: Skip a single VM_NONE mapping when checking the stack gap > > prev = vma->vm_prev; > + if (prev && !(prev->vm_flags & (VM_READ | VM_WRITE | VM_EXEC))) > + prev = prev->vm_prev; > if (prev && prev->vm_end > gap_addr) {
Do we just want to ignore the user-supplied guard mapping, or do we want to say "if the user does a guard mapping, we use that *instead* of our stack gap"?
IOW, instead of "prev = prev->vm_prev;" and continuing, maybe we want to just return "ok".
> Subject: [2/2] mmap: Avoid mapping anywhere within the full stack extent if finite
This is good thinking, but no, I don't think the "if finite" is right.
I've seen people use "really big values" as replacement for RLIM_INIFITY, for various reasons.
We've had huge confusion about RLIM_INFINITY over the years - look for things like COMPAT_RLIM_OLD_INFINITY to see the kinds of confusions we've had.
Some people just use MAX_LONG etc, which is *not* the same as RLIM_INFINITY, but in practice ends up doing the same thing. Yadda yadda.
So I'm personally leery of checking and depending on "exactly RLIM_INIFITY", because I've seen it go wrong so many times.
And I think your second patch breaks that "use a really large value to approximate infinity" case that definitely has existed as a pattern.
Linus
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