Messages in this thread | | | From | Marco Elver <> | Date | Thu, 9 Dec 2021 21:33:01 +0100 | Subject | Re: randomize_kstack: To init or not to init? |
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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.
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