From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/4719 Path: news.gmane.org!not-for-mail From: Laurent Bercot Newsgroups: gmane.linux.lib.musl.general Subject: Re: Transition path for removing lazy init of thread pointer Date: Tue, 25 Mar 2014 06:35:02 +0000 Message-ID: <53312396.5060307@skarnet.org> References: <20140324174915.GA1263@brightrain.aerifal.cx> <20140324230405.GA23163@brightrain.aerifal.cx> <5330C769.6080304@skarnet.org> <20140325015531.GB23474@brightrain.aerifal.cx> Reply-To: musl@lists.openwall.com NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Trace: ger.gmane.org 1395729302 4142 80.91.229.3 (25 Mar 2014 06:35:02 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Tue, 25 Mar 2014 06:35:02 +0000 (UTC) To: musl@lists.openwall.com Original-X-From: musl-return-4723-gllmg-musl=m.gmane.org@lists.openwall.com Tue Mar 25 07:35:12 2014 Return-path: Envelope-to: gllmg-musl@plane.gmane.org Original-Received: from mother.openwall.net ([195.42.179.200]) by plane.gmane.org with smtp (Exim 4.69) (envelope-from ) id 1WSKx6-0007t4-2v for gllmg-musl@plane.gmane.org; Tue, 25 Mar 2014 07:35:12 +0100 Original-Received: (qmail 7897 invoked by uid 550); 25 Mar 2014 06:35:11 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: Original-Received: (qmail 7887 invoked from network); 25 Mar 2014 06:35:11 -0000 X-SourceIP: 109.255.124.30 User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.4.0 In-Reply-To: <20140325015531.GB23474@brightrain.aerifal.cx> Xref: news.gmane.org gmane.linux.lib.musl.general:4719 Archived-At: On 25/03/2014 01:55, Rich Felker wrote: > The mandatory syscall is set_thread_area or equivalent, e.g. > arch_prctl on x86_64. It's there because most archs need a syscall to > set the thread pointer used for accessing TLS. Even in single-threaded > programs, there are reasons one may want to have it. > > The big reason is that, on most archs, stack protector's canary value > is stored at a fixed offset from the thread pointer rather than in a > global, so stack protector can't work without the thread pointer being > initialized. Up to now we've tried to detect whether stack protector > is used based on symbol references to __stack_chk_fail, but this check > gives a false negative (and thus crashing programs) if gcc optimizes > out the check to __stack_chk_fail but not the load of the canary, e.g. > in the program: int main() { exit(0); } That's a good reason indeed. I take it you're still hell-bent against compile-time options ? Because a musl compile-time option "I don't want this musl to support stack protector, yes I know it will crash programs compiled with it, but I'm a big boy and know what I'm doing" would be great for OCD people like me who like their strace clean. :) > The other main reason is that lazy initialization is a lot more > expensive at runtime. That's not a good reason for single-threaded programs. > So despite always initializing the thread pointer kinda looking like > "bloat" from a minimal-program standpoint, it's really a major step > forward in debloating and simplifying lots of code. I totally understand and approve for multi-threaded programs and programs using stack protection. I just wish there were a special optimization for "int main() { return 0; }". -- Laurent