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Date:   Tue, 9 Jun 2020 11:46:40 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Alexander Popov <alex.popov@linux.com>
Cc:     Emese Revfy <re.emese@gmail.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
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        Andrew Morton <akpm@linux-foundation.org>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Thiago Jung Bauermann <bauerman@linux.ibm.com>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Jessica Yu <jeyu@kernel.org>,
        Sven Schnelle <svens@stackframe.org>,
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        Catalin Marinas <catalin.marinas@arm.com>,
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Subject: Re: [PATCH 2/5] gcc-plugins/stackleak: Use asm instrumentation to
 avoid useless register saving
Message-ID: <202006091143.AD1A662@keescook>
References: <20200604134957.505389-1-alex.popov@linux.com>
 <20200604134957.505389-3-alex.popov@linux.com>
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On Thu, Jun 04, 2020 at 04:49:54PM +0300, Alexander Popov wrote:
> Let's improve the instrumentation to avoid this:
> 
> 1. Make stackleak_track_stack() save all register that it works with.
> Use no_caller_saved_registers attribute for that function. This attribute
> is available for x86_64 and i386 starting from gcc-7.
> 
> 2. Insert calling stackleak_track_stack() in asm:
>   asm volatile("call stackleak_track_stack" :: "r" (current_stack_pointer))
> Here we use ASM_CALL_CONSTRAINT trick from arch/x86/include/asm/asm.h.
> The input constraint is taken into account during gcc shrink-wrapping
> optimization. It is needed to be sure that stackleak_track_stack() call is
> inserted after the prologue of the containing function, when the stack
> frame is prepared.

Very cool; nice work!

> +static void add_stack_tracking(gimple_stmt_iterator *gsi)
> +{
> +	/*
> +	 * The 'no_caller_saved_registers' attribute is used for
> +	 * stackleak_track_stack(). If the compiler supports this attribute for
> +	 * the target arch, we can add calling stackleak_track_stack() in asm.
> +	 * That improves performance: we avoid useless operations with the
> +	 * caller-saved registers in the functions from which we will remove
> +	 * stackleak_track_stack() call during the stackleak_cleanup pass.
> +	 */
> +	if (lookup_attribute_spec(get_identifier("no_caller_saved_registers")))
> +		add_stack_tracking_gasm(gsi);
> +	else
> +		add_stack_tracking_gcall(gsi);
> +}

The build_for_x86 flag is only ever used as an assert() test against
no_caller_saved_registers, but we're able to test for that separately.
Why does the architecture need to be tested? (i.e. when this flag
becomes supported o other architectures, why must it still be x86-only?)

-- 
Kees Cook