linux-hardening - Re: [RESEND PATCH v2] params: Annotate struct module_param_attrs with __counted

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Message-ID: <202409170436.C3C6E7F7A@keescook>
Date: Tue, 17 Sep 2024 04:43:32 -0700
From: Kees Cook <kees@...nel.org>
To: Bill Wendling <morbo@...gle.com>
Cc: Thorsten Blum <thorsten.blum@...lux.com>,
	Nathan Chancellor <nathan@...nel.org>, gustavoars@...nel.org,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	mcgrof@...nel.org, linux-hardening@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v2] params: Annotate struct module_param_attrs
 with __counted_by()

On Mon, Sep 16, 2024 at 02:45:47AM -0700, Bill Wendling wrote:
> The 4294967295 simply means "I don't know." There's probably a bug in
> the size calculation. I'll look into it.

I was able to build a minimized PoC, if that's helpful:

https://godbolt.org/z/qohGd5xh1


#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

struct variable {
        int a;
        int b;
        int length;
        short array[] __attribute__((counted_by(length)));
};

struct bucket {
        int a;
        struct variable *growable;
        int b;
};

int main(int argc, char *argv[])
{
        struct bucket *p;
        struct variable *v;

        p = malloc(sizeof(*p));
        v = malloc(sizeof(*p->growable) + sizeof(*p->growable->array) * 32);
        v->length = 32;


        printf("%zu\n", __builtin_dynamic_object_size(v->array, 1));

        p->growable = v;
        printf("%zu\n", __builtin_dynamic_object_size(p->growable->array, 1));

        return 0;
}


GCC shows 64 64, but Clang shows 64 0.

-- 
Kees Cook