https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=241905
--- Comment #8 from commit-h...@freebsd.org --- A commit references this bug: Author: kevans Date: Wed Nov 13 02:14:17 UTC 2019 New revision: 354669 URL: https://svnweb.freebsd.org/changeset/base/354669 Log: ssp: add a priority to the __stack_chk_guard constructor First, this commit is a NOP on GCC <= 4.x; this decidedly doesn't work cleanly on GCC 4.2, and it will be gone soon anyways so I chose not to dump time into figuring out if there's a way to make it work. xtoolchain-gcc, clocking in as GCC6, can cope with it just fine and later versions are also generally ok with the syntax. I suspect very few users are running GCC4.2 built worlds and also experiencing potential fallout from the status quo. For dynamically linked applications, this change also means very little. rtld will run libc ctors before most others, so the situation is approximately a NOP for these as well. The real cause for this change is statically linked applications doing almost questionable things in their constructors. qemu-user-static, for instance, creates a thread in a global constructor for their async rcu callbacks. In general, this works in other places- - On OpenBSD, __stack_chk_guard is stored in an .openbsd.randomdata section that's initialized by the kernel in the static case, or ld.so in the dynamic case - On Linux, __stack_chk_guard is apparently stored in TLS and such a problem is circumvented there because the value is presumed stable in the new thread. On FreeBSD, the rcu thread creation ctor and __guard_setup are both unmarked priority. qemu-user-static spins up the rcu thread prior to __guard_setup which starts making function calls- some of these are sprinkled with the canary. In the middle of one of these functions, __guard_setup is invoked in the main thread and __stack_chk_guard changes- qemu-user-static is promptly terminated for an SSP violation that didn't actually happen. This is not an all-too-common problem. We circumvent it here by giving the __stack_chk_guard constructor a solid priority. 200 was chosen because that gives static applications ample range (down to 101) for working around it if they really need to. I suspect most applications will "just work" as expected- the default/non-prioritized flavor of __constructor__ functions run last, and the canary is generally not expected to change as of this point at the very least. This took approximately three weeks of spare time debugging to pin down. PR: 241905 Changes: head/lib/libc/secure/stack_protector.c -- You are receiving this mail because: You are the assignee for the bug. _______________________________________________ freebsd-toolchain@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-toolchain To unsubscribe, send any mail to "freebsd-toolchain-unsubscr...@freebsd.org"
