Date: Thu, 20 Jul 2023 21:39:01 +0000 From: Brooks Davis <brooks@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: src-committers@freebsd.org, dev-commits-src-all@freebsd.org, dev-commits-src-main@freebsd.org Subject: Re: git: 21e45c30c35c - main - mmap(MAP_STACK): on stack grow, use original protection Message-ID: <ZLmpdQbqG5dQudZ9@spindle.one-eyed-alien.net> In-Reply-To: <ZLmd0dOp6qbbBf-N@kib.kiev.ua> References: <202307201412.36KECDSU084918@gitrepo.freebsd.org> <ZLmV0wQ1mgkiSbwK@spindle.one-eyed-alien.net> <ZLmd0dOp6qbbBf-N@kib.kiev.ua>
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On Thu, Jul 20, 2023 at 11:49:21PM +0300, Konstantin Belousov wrote: > On Thu, Jul 20, 2023 at 08:15:15PM +0000, Brooks Davis wrote: > > On Thu, Jul 20, 2023 at 02:12:13PM +0000, Konstantin Belousov wrote: > > > The branch main has been updated by kib: > > > > > > URL: https://cgit.FreeBSD.org/src/commit/?id=21e45c30c35c9aa732073f725924caf581c93460 > > > > > > commit 21e45c30c35c9aa732073f725924caf581c93460 > > > Author: Konstantin Belousov <kib@FreeBSD.org> > > > AuthorDate: 2023-07-19 11:05:32 +0000 > > > Commit: Konstantin Belousov <kib@FreeBSD.org> > > > CommitDate: 2023-07-20 14:11:42 +0000 > > > > > > mmap(MAP_STACK): on stack grow, use original protection > > > > > > If mprotect(2) changed protection in the bottom of the currently grown > > > stack region, currently the changed protection would be used for the > > > stack grow on next fault. This is arguably unexpected. > > > > > > Store the original protection for the entry at mmap(2) time in the > > > offset member of the gap vm_map_entry, and use it for protection of the > > > grown stack region. > > > > It occured to me to wonder how this interacts with > > __enable_execute_stack(). I think it's ok so long as the compiler > > always emits __enable_execute_stack() for each trampoline and doesn't > > depend on extentions to the stack being executable from the first > > trampoline creation, but I don't know where to look to verify that's the > > case. > Even on first trampoline creation, compiler cannot know how much was the > stack grown already. So the generated code cannot be sure that it changing > the protection for the bottom stack page. Then, it cannot work even for > more usual stack grow organization. That makes sense. Thanks for the clarification. -- Brooks
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