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Date:   Mon, 30 Aug 2021 13:16:41 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     linux-kernel@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Keith Packard <keithp@keithp.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        clang-built-linux@googlegroups.com,
        linux-hardening@vger.kernel.org, llvm@lists.linux.dev
Subject: Re: [PATCH v3 0/5] Enable -Warray-bounds and -Wzero-length-bounds
Message-ID: <202108301314.22B3CB015C@keescook>
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On Mon, Aug 30, 2021 at 11:44:54AM -0700, Nathan Chancellor wrote:
> arch/powerpc/kernel/signal_32.c:780:2: error: array index 3 is past the end of the array (which contains 1 element) [-Werror,-Warray-bounds]
>         unsafe_put_sigset_t(&frame->uc.uc_sigmask, oldset, failed);
>         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Or is this a Clang DCE failure?

#define unsafe_put_compat_sigset(compat, set, label) do {               \
        compat_sigset_t __user *__c = compat;                           \
        const sigset_t *__s = set;                                      \
                                                                        \
        switch (_NSIG_WORDS) {                                          \
        case 4:                                                         \
                unsafe_put_user(__s->sig[3] >> 32, &__c->sig[7], label);        \
                unsafe_put_user(__s->sig[3], &__c->sig[6], label);      \
                fallthrough;                                            \
        case 3:                                                         \
                unsafe_put_user(__s->sig[2] >> 32, &__c->sig[5], label);        \
                unsafe_put_user(__s->sig[2], &__c->sig[4], label);      \
                fallthrough;                                            \
        case 2:                                                         \
                unsafe_put_user(__s->sig[1] >> 32, &__c->sig[3], label);        \
                unsafe_put_user(__s->sig[1], &__c->sig[2], label);      \
                fallthrough;                                            \
        case 1:                                                         \
                unsafe_put_user(__s->sig[0] >> 32, &__c->sig[1], label);        \
                unsafe_put_user(__s->sig[0], &__c->sig[0], label);      \
        }                                                               \
} while (0)

if "set" has only 1 element, then _NSIG_WORDS must be 1. The warnings
are coming from cases 4 and 3. (But why not 2, which would also access
beyond the end?)

-- 
Kees Cook