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Re: [Qemu-devel] [PATCH 1/1] configure: use appropriate code fragment fo
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-devel] [PATCH 1/1] configure: use appropriate code fragment for -fstack-protector checks |
Date: |
Thu, 12 Nov 2015 09:41:49 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) |
Rodrigo Rebello <address@hidden> writes:
> The check for stack-protector support consisted in compiling and linking
> the test program below (output by function write_c_skeleton()) with the
> compiler flag -fstack-protector-strong first and then with
> -fstack-protector-all if the first one failed to work:
>
> int main(void) { return 0; }
>
> This caused false positives when using certain toolchains in which the
> compiler accepted -fstack-protector-strong but no support was provided
> by the C library, since for this stack-protector variant the compiler
> emits canary code only for functions that meet specific conditions
> (local arrays, memory references to local variables, etc.) and the code
> fragment under test included none of them (hence no stack protection
> code generated, no link failure).
>
> This fix changes the test program used for -fstack-protector checks to
> include a function that meets conditions which cause the compiler to
> generate canary code in all variants.
>
> Signed-off-by: Rodrigo Rebello <address@hidden>
> ---
> configure | 18 ++++++++++++++++++
> 1 file changed, 18 insertions(+)
>
> diff --git a/configure b/configure
> index 46fd8bd..c3d9592 100755
> --- a/configure
> +++ b/configure
> @@ -1486,6 +1486,24 @@ for flag in $gcc_flags; do
> done
>
> if test "$stack_protector" != "no"; then
> + cat > $TMPC << EOF
> +void foo(const char *c);
> +
> +void foo(const char *c)
> +{
> + char arr[64], *p;
> + for (p = arr; *c; c++, p++) {
> + *p = *c;
> + }
> +}
> +
> +int main(void)
> +{
> + char c[] = "";
> + foo(c);
Why not simply foo("")?
Could the optimizer optimize away the pattern that triggers the canary?
To protect against that possibility, we could use
int main(int argc, char *argv[])
{
foo(argv[0]);
}
> + return 0;
> +}
> +EOF
> gcc_flags="-fstack-protector-strong -fstack-protector-all"
> sp_on=0
> for flag in $gcc_flags; do