From owner-freebsd-hackers@freebsd.org Wed Mar 31 02:28:23 2021 Return-Path: Delivered-To: freebsd-hackers@mailman.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.nyi.freebsd.org (Postfix) with ESMTP id 989345B7E4C for ; Wed, 31 Mar 2021 02:28:23 +0000 (UTC) (envelope-from asomers@gmail.com) Received: from mailman.nyi.freebsd.org (unknown [127.0.1.3]) by mx1.freebsd.org (Postfix) with ESMTP id 4F99Gb2PM7z4RMk for ; Wed, 31 Mar 2021 02:28:23 +0000 (UTC) (envelope-from asomers@gmail.com) Received: by mailman.nyi.freebsd.org (Postfix) id 506AE5B7E4B; Wed, 31 Mar 2021 02:28:23 +0000 (UTC) Delivered-To: hackers@mailman.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.nyi.freebsd.org (Postfix) with ESMTP id 502EB5B7E4A for ; Wed, 31 Mar 2021 02:28:23 +0000 (UTC) (envelope-from asomers@gmail.com) Received: from mail-lj1-f169.google.com (mail-lj1-f169.google.com [209.85.208.169]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256 client-signature RSA-PSS (2048 bits) client-digest SHA256) (Client CN "smtp.gmail.com", Issuer "GTS CA 1O1" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 4F99Gb1lYbz4RV9 for ; Wed, 31 Mar 2021 02:28:22 +0000 (UTC) (envelope-from asomers@gmail.com) Received: by mail-lj1-f169.google.com with SMTP id 184so22032172ljf.9 for ; Tue, 30 Mar 2021 19:28:22 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=zRWxaK/SAqSBT2BLLzcaev/CvRCTEAeUw0JLC0P75VU=; b=F9QSVblvyr0TUJ/m2/BmqSEGROMduDnFkqMUeYYLKCDlonqyV0xik2zLOXoC/65fhv zLDz7yFIvPTDBUqkU451BpoFDuvtHguLTG1Dd67HM+RFh8CkpjM+11H16LDw/Gcj+AuZ kgybt4qcThYYKfFnJgiktVR6qv3MJcIEN+ZeOwkLJWT4wn80GdpEC8YZkKU/Hh748Leu Ia0qnG9X7iI6a5HOr+EPi9a2ZVNXp7uaUd4GtRUY/Jm/ifVfQQqxuCaEFmXxff9l6/5U URle1/54+pirnsrIgIPlT9UzrTv96e5LH6XJc+8+a6xbS16X1lYO+s7VgTADGp6PwyrS 944g== X-Gm-Message-State: AOAM533YBeAqPzgb8VjWqO+sBs6jJbgS1nacGWCSQ9f/8KJWXIHrEWWP MgmgzCq/5fhCSDbzXvqEsSApW3J7ecwXSpcjHMU= X-Google-Smtp-Source: ABdhPJyO+NA3km34qjai3DM/RO6SR7132eLyC8opRIjAJNhYklUovuqPheNMIOhH3qmHHx5mkrhZdzIGQJ16PXEUe0o= X-Received: by 2002:a2e:7619:: with SMTP id r25mr615020ljc.408.1617157700823; Tue, 30 Mar 2021 19:28:20 -0700 (PDT) MIME-Version: 1.0 References: In-Reply-To: From: Alan Somers Date: Tue, 30 Mar 2021 20:28:09 -0600 Message-ID: Subject: Re: How does the stack's guard page work on amd64? To: Konstantin Belousov Cc: "freebsd-hackers@freebsd.org" X-Rspamd-Queue-Id: 4F99Gb1lYbz4RV9 X-Spamd-Bar: ---- Authentication-Results: mx1.freebsd.org; none X-Spamd-Result: default: False [-4.00 / 15.00]; REPLY(-4.00)[] Content-Type: text/plain; charset="UTF-8" X-Content-Filtered-By: Mailman/MimeDel 2.1.34 X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: Technical discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 31 Mar 2021 02:28:23 -0000 On Tue, Mar 30, 2021 at 3:35 AM Konstantin Belousov 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? 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(0x7fffdfffe000,0x1000,0x3,0x1012,0xffffffff,0) mprotect(0x7fffdfffe000,0x1000,0) 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 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 ? -Alan