Messages in this thread | | | Date | Thu, 9 Dec 2021 14:16:16 -0600 | From | Segher Boessenkool <> | Subject | Re: randomize_kstack: To init or not to init? |
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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.
Segher
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