* Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> gcc-11 warns about using string operations on pointers that are
> defined at compile time as offsets from a NULL pointer. Unfortunately
> that also happens on the result of fix_to_virt(), which is a
> compile-time constant for a constantn input:
>
> arch/x86/kernel/tboot.c: In function 'tboot_probe':
> arch/x86/kernel/tboot.c:70:13: error: '__builtin_memcmp_eq' specified bound 16 exceeds source size 0 [-Werror=stringop-overread]
> 70 | if (memcmp(&tboot_uuid, &tboot->uuid, sizeof(tboot->uuid))) {
> | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> I hope this can get addressed in gcc-11 before the release.
>
> As a workaround, split up the tboot_probe() function in two halves
> to separate the pointer generation from the usage. This is a bit
> ugly, and hopefully gcc understands that the code is actually correct
> before it learns to peek into the noinline function.
>
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99578
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
> arch/x86/kernel/tboot.c | 44 ++++++++++++++++++++++++-----------------
> 1 file changed, 26 insertions(+), 18 deletions(-)
>
> diff --git a/arch/x86/kernel/tboot.c b/arch/x86/kernel/tboot.c
> index 4c09ba110204..f9af561c3cd4 100644
> --- a/arch/x86/kernel/tboot.c
> +++ b/arch/x86/kernel/tboot.c
> @@ -49,6 +49,30 @@ bool tboot_enabled(void)
> return tboot != NULL;
> }
>
> +/* noinline to prevent gcc from warning about dereferencing constant fixaddr */
> +static noinline __init bool check_tboot_version(void)
> +{
> + if (memcmp(&tboot_uuid, &tboot->uuid, sizeof(tboot->uuid))) {
> + pr_warn("tboot at 0x%llx is invalid\n", boot_params.tboot_addr);
> + return false;
> + }
> +
> + if (tboot->version < 5) {
> + pr_warn("tboot version is invalid: %u\n", tboot->version);
> + return false;
> + }
> +
> + pr_info("found shared page at phys addr 0x%llx:\n",
> + boot_params.tboot_addr);
> + pr_debug("version: %d\n", tboot->version);
> + pr_debug("log_addr: 0x%08x\n", tboot->log_addr);
> + pr_debug("shutdown_entry: 0x%x\n", tboot->shutdown_entry);
> + pr_debug("tboot_base: 0x%08x\n", tboot->tboot_base);
> + pr_debug("tboot_size: 0x%x\n", tboot->tboot_size);
> +
> + return true;
> +}
> +
> void __init tboot_probe(void)
> {
> /* Look for valid page-aligned address for shared page. */
> @@ -66,25 +90,9 @@ void __init tboot_probe(void)
>
> /* Map and check for tboot UUID. */
> set_fixmap(FIX_TBOOT_BASE, boot_params.tboot_addr);
> - tboot = (struct tboot *)fix_to_virt(FIX_TBOOT_BASE);
> - if (memcmp(&tboot_uuid, &tboot->uuid, sizeof(tboot->uuid))) {
> - pr_warn("tboot at 0x%llx is invalid\n", boot_params.tboot_addr);
> + tboot = (void *)fix_to_virt(FIX_TBOOT_BASE);
> + if (!check_tboot_version())
> tboot = NULL;
> - return;
> - }
> - if (tboot->version < 5) {
> - pr_warn("tboot version is invalid: %u\n", tboot->version);
> - tboot = NULL;
> - return;
> - }
> -
> - pr_info("found shared page at phys addr 0x%llx:\n",
> - boot_params.tboot_addr);
> - pr_debug("version: %d\n", tboot->version);
> - pr_debug("log_addr: 0x%08x\n", tboot->log_addr);
> - pr_debug("shutdown_entry: 0x%x\n", tboot->shutdown_entry);
> - pr_debug("tboot_base: 0x%08x\n", tboot->tboot_base);
> - pr_debug("tboot_size: 0x%x\n", tboot->tboot_size);
This is indeed rather ugly - and the other patch that removes a debug
check seems counterproductive as well.
Do we know how many genuine bugs -Wstringop-overread-warning has
caught or is about to catch?
I.e. the real workaround might be to turn off the -Wstringop-overread-warning,
until GCC-11 gets fixed?
Thanks,
Ingo
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