From: Kees Cook <email@example.com>
To: Nick Desaulniers <firstname.lastname@example.org>
Cc: George Burgess IV <email@example.com>,
Miguel Ojeda <firstname.lastname@example.org>,
Nathan Chancellor <email@example.com>,
Subject: Re: [PATCH v6 1/4] Compiler Attributes: Add __pass_object_size for Clang
Date: Thu, 3 Feb 2022 12:58:17 -0800 [thread overview]
Message-ID: <202202031247.4F3AC598@keescook> (raw)
On Thu, Feb 03, 2022 at 12:18:24PM -0800, Nick Desaulniers wrote:
> On Thu, Feb 3, 2022 at 9:33 AM Kees Cook <firstname.lastname@example.org> wrote:
> > In order to gain greater visibility to type information when using
> > __builtin_object_size(), Clang has a function attribute "pass_object_size"
> > that will make size information available for marked arguments in
> > a function by way of implicit additional function arguments that are
> > then wired up the __builtin_object_size().
> > This is needed to implement FORTIFY_SOURCE in Clang, as a workaround
> > to Clang's __builtin_object_size() having limited visibility into types
> > across function calls (even inlines).
> > Since any usage must also be const, include it in the macro.
> I really don't like hiding the qualifier in the argument-attribute
> macro like that. I'd rather we be more explicit about changing the
> function interfaces in include/linux/fortify-string.h.
It seemed redundant to have it separate when it would always be needed.
In v5 I had the const in the BOS (now POS) macro, but realized that this
was silly since it would always need to be that way for __pos.
> For instance, in patch 4/4, let's take a look at this hunk:
> -__FORTIFY_INLINE char *strncpy(char *p, const char *q, __kernel_size_t size)
> +__FORTIFY_INLINE __diagnose_as(__builtin_strncpy, 1, 2, 3)
> +char *strncpy(char * POS p, const char *q, __kernel_size_t size)
> manually expanded, this has changed the qualifiers on the type of the
> first parameter from `char *` to `char * const`.
Yup, that's expected, and I wanted to tie it strictly to the expansion
of __pass_object_size since otherwise GCC would gain the "const" bit
(which technically shouldn't matter, but this whole series has been
nothing but corner case after corner case, and I didn't want to risk
> I think it might be helpful to quote the docs
> >> Additionally, any parameter that pass_object_size is applied to must be marked const at its function’s definition.
> One thing that's concerning to me is though:
> >> It is an error to take the address of a function with pass_object_size on any of its parameters.
> Surely the kernel has indirect calls to some of these functions
> somewhere? Is that just an issue for C++ name-mangling perhaps?
AFAIU, this shouldn't be a problem for any of these. Nothing is trying
to take memcpy, memset, etc by address. The macro-ified version of this
change proved that out. :)
next prev parent reply other threads:[~2022-02-03 20:58 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-03 17:33 [PATCH v6 0/4] fortify: Add Clang support Kees Cook
2022-02-03 17:33 ` [PATCH v6 1/4] Compiler Attributes: Add __pass_object_size for Clang Kees Cook
2022-02-03 20:18 ` Nick Desaulniers
2022-02-03 20:58 ` Kees Cook [this message]
2022-02-03 22:01 ` Nick Desaulniers
2022-02-04 0:29 ` Kees Cook
2022-02-03 17:33 ` [PATCH v6 2/4] Compiler Attributes: Add __overloadable " Kees Cook
2022-02-03 20:26 ` Nick Desaulniers
2022-02-03 21:04 ` Kees Cook
2022-02-03 22:11 ` Nick Desaulniers
2022-02-04 0:26 ` Kees Cook
2022-02-04 0:58 ` Nick Desaulniers
2022-02-04 1:07 ` Miguel Ojeda
2022-02-03 17:33 ` [PATCH v6 3/4] Compiler Attributes: Add __diagnose_as " Kees Cook
2022-02-03 20:28 ` Nick Desaulniers
2022-02-03 17:33 ` [PATCH v6 4/4] fortify: Add Clang support Kees Cook
2022-02-03 20:37 ` Nick Desaulniers
2022-02-03 21:26 ` Kees Cook
2022-02-03 17:47 ` [PATCH v6 0/4] " Miguel Ojeda
2022-02-03 19:57 ` Kees Cook
2022-02-03 21:12 ` Miguel Ojeda
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