Date: Wed, 31 Mar 2021 14:21:16 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Alan Somers <asomers@freebsd.org> Cc: "freebsd-hackers@freebsd.org" <hackers@freebsd.org> Subject: Re: How does the stack's guard page work on amd64? Message-ID: <YGRbLLeyqsYa9fp7@kib.kiev.ua> In-Reply-To: <CAOtMX2gM9n%2BnYEErtv_FmQkJAB5JJ4tpXGydB6oo8qoEjq57yg@mail.gmail.com> References: <CAOtMX2i5d0c9E=W=S6aKp1j5JczaaTqKDX8kW=2NqF=i35dWog@mail.gmail.com> <YGLwv%2BKkmhxeeJUp@kib.kiev.ua> <CAOtMX2gM9n%2BnYEErtv_FmQkJAB5JJ4tpXGydB6oo8qoEjq57yg@mail.gmail.com>
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On Tue, Mar 30, 2021 at 08:28:09PM -0600, Alan Somers wrote: > On Tue, Mar 30, 2021 at 3:35 AM Konstantin Belousov <kostikbel@gmail.com> > wrote: > > > On Mon, Mar 29, 2021 at 11:06:36PM -0600, Alan Somers wrote: > > > Rust tries to detect stack overflow and handles it differently than other > > > segfaults, but it's currently broken on FreeBSD/amd64. I've got a patch > > > that fixes the problem, but I would like someone to confirm my reasoning. > > > > > > It seems like FreeBSD's main thread stacks include a guard page at the > > > bottom. However, when Rust tries to create its own guard page (by > > > re-mmap()ping and mprotect()ing it), it seems like FreeBSD's guard page > > > automatically moves up into the un-remapped region. At least, that's how > > > it behaves, based on the addresses that segfault. Is that correct? > > Show the facts. For instance, procstat -v (and a note which > > mapping was established by runtime for the 'guard') would tell the whole > > story. > > > > My guess would be that procctl(PROC_STACKGAP_CTL, &PROC_STACKGAP_DISABLE) > > would be enough. Cannot tell without specific data. > > > > > > > > For other threads, Rust doesn't try to remap the guard page, it just > > relies > > > on the guard page created by libthr in _thr_stack_alloc. > > > > > > Finally, what changed in between FreeBSD 10.3 and 11.4? Rust's stack > > > overflow detection worked in 10.3. > > > > > > -Alan > > > _______________________________________________ > > > freebsd-hackers@freebsd.org mailing list > > > https://lists.freebsd.org/mailman/listinfo/freebsd-hackers > > > To unsubscribe, send any mail to " > > freebsd-hackers-unsubscribe@freebsd.org" > > > > Here is the relevant portion of procstat -v for a test program built with > the buggy rustc: > 651 0x801554000 0x80155d000 rw- 0 17 3 0 ----- df > 651 0x801600000 0x801e00000 rw- 30 30 1 0 ----- df > 651 0x7fffdfffd000 0x7fffdfffe000 --- 0 0 0 0 ----- -- > 651 0x7fffdfffe000 0x7fffdffff000 --- 0 0 0 0 ----- -- > <--- What Rustc thinks is the guard page > 651 0x7fffdffff000 0x7fffe0000000 --- 0 0 0 0 ----- -- > <--- Where did this come from? This is the stack grow area, occupied by 'elastic' guard entry. It serves two purposes: 1. it keeps the space, preventing other non-fixed mappings from selecting the grow area for mapping. 2. it prevents stack from growing down to the next mapping below it, preventing issues like StackClash. See mmap(2) esp. MAP_STACK part of it. > 651 0x7fffe0000000 0x7fffe001e000 rw- 30 30 1 0 ---D- df > 651 0x7fffe001e000 0x7fffe003e000 rw- 32 32 1 0 ---D- df > > Rustc tries to create that guard page by finding the base address of the > stack, reallocating one page, then mprotect()ing it, like this: > mmap(0x7fffdfffe000,0x1000,0x3<PROT_READ|PROT_WRITE>,0x1012<MAP_PRIVATE|MAP_FIXED|MAP_ANON>,0xffffffff,0) > mprotect(0x7fffdfffe000,0x1000,0<PROT_NONE>) > > If I patch rustc to not attempt to allocate a guard page, then its memory > map looks like this. Notice that 0x7fffdffff000 is now accessible It is accessible because stack grown down into this address. > 662 0x801531000 0x80155b000 rw- 3 17 3 0 ----- df > 662 0x801600000 0x801e00000 rw- 30 30 1 0 ----- df > 662 0x7fffdfffd000 0x7fffdfffe000 --- 0 0 0 0 ----- -- > 662 0x7fffdfffe000 0x7fffdffff000 --- 0 0 0 0 ----- -- > 662 0x7fffdffff000 0x7fffe001e000 rw- 31 31 1 0 ---D- df > 662 0x7fffe001e000 0x7fffe003e000 rw- 32 32 1 0 ---D- df > > So the real question is, why does 0x7fffdffff000 become protected when > rustc protects 0x7fffdfffe000 ? See above. As I said in earlier response, if you want fully shrinkable stack guard, set procctl(PROC_STACKGAP_CTL, &PROC_STACKGAP_DISABLE) during runtime initialization. Or better, do not create custom guard page at all, relying on system guard.
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