Messages in this thread | | | From | Marco Elver <> | Date | Mon, 20 Dec 2021 08:00:00 +0100 | Subject | Re: randomize_kstack: To init or not to init? |
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On Thu, 9 Dec 2021 at 21:33, Marco Elver <elver@google.com> wrote: > On Thu, 9 Dec 2021 at 21:19, Segher Boessenkool <segher@kernel.crashing.org> wrote: > > On Thu, Dec 09, 2021 at 10:58:01AM +0100, Marco Elver wrote: > > > Clang supports CONFIG_INIT_STACK_ALL_ZERO, which appears to be the > > > default since dcb7c0b9461c2, which is why this came on my radar. And > > > Clang also performs auto-init of allocas when auto-init is on > > > (https://reviews.llvm.org/D60548), with no way to skip. As far as I'm > > > aware, GCC 12's upcoming -ftrivial-auto-var-init= doesn't yet auto-init > > > allocas. > > > > The space allocated by alloca is not an automatic variable, so of course > > it is not affected by this compiler flag. And it should not, this flag > > is explicitly for *small fixed-size* stack variables (initialising > > others can be much too expensive). > > > > > C. Introduce a new __builtin_alloca_uninitialized(). > > > > That is completely backwards. That is the normal behaviour of alloca > > already. Also you can get __builtin_alloca inserted by the compiler > > (for a variable length array for example), and you typically do not want > > those initialised either, for the same reasons. > > You're right, if we're strict about it, initializing allocas is > technically out-of-scope of that feature. > > So, option D: Add a param to control this, and probably it shouldn't > do it by default. Let's see how far that gets then.
Just an update: after some discussion, the Clang side says that alloca() is in scope, because the intent is that trivially initialized "automatic stack storage" should be handled by ftrivial-auto-var-init. And alloca() is automatic stack storage: https://www.gnu.org/software/libc/manual/html_node/Variable-Size-Automatic.html
So currently it looks like the builtin is the only solution in this case.
Thanks.
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