linux-kernel - Re: [PATCH] samples/ftrace: mark my

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Date:   Mon, 16 Nov 2020 11:39:31 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Colin Ian King <colin.king@...onical.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] samples/ftrace: mark my_tramp[12]? global

On Fri, 13 Nov 2020 10:34:14 -0800
Sami Tolvanen <samitolvanen@...gle.com> wrote:

> my_tramp[12]? are declared as global functions in C, but they are not
> marked global in the inline assembly definition. This mismatch confuses
> Clang's Control-Flow Integrity checking. Fix the definitions by adding
> .globl.
> 

Actually, since that function is not really global, would it work if you
removed the "extern" from the my_tramp declaration?

In other words, is there a way to tell C that a function is declared in an
inline assembly block?

-- Steve


> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  samples/ftrace/ftrace-direct-modify.c | 2 ++
>  samples/ftrace/ftrace-direct-too.c    | 1 +
>  samples/ftrace/ftrace-direct.c        | 1 +
>  3 files changed, 4 insertions(+)
> 
> diff --git a/samples/ftrace/ftrace-direct-modify.c b/samples/ftrace/ftrace-direct-modify.c
> index c13a5bc5095b..5b9a09957c6e 100644
> --- a/samples/ftrace/ftrace-direct-modify.c
> +++ b/samples/ftrace/ftrace-direct-modify.c
> @@ -21,6 +21,7 @@ static unsigned long my_ip = (unsigned long)schedule;
>  asm (
>  "	.pushsection    .text, \"ax\", @progbits\n"
>  "	.type		my_tramp1, @function\n"
> +"	.globl		my_tramp1\n"
>  "   my_tramp1:"
>  "	pushq %rbp\n"
>  "	movq %rsp, %rbp\n"
> @@ -29,6 +30,7 @@ asm (
>  "	.size		my_tramp1, .-my_tramp1\n"
>  "	ret\n"
>  "	.type		my_tramp2, @function\n"
> +"	.globl		my_tramp2\n"
>  "   my_tramp2:"
>  "	pushq %rbp\n"
>  "	movq %rsp, %rbp\n"
> diff --git a/samples/ftrace/ftrace-direct-too.c b/samples/ftrace/ftrace-direct-too.c
> index d5c5022be664..3f0079c9bd6f 100644
> --- a/samples/ftrace/ftrace-direct-too.c
> +++ b/samples/ftrace/ftrace-direct-too.c
> @@ -16,6 +16,7 @@ extern void my_tramp(void *);
>  asm (
>  "	.pushsection    .text, \"ax\", @progbits\n"
>  "	.type		my_tramp, @function\n"
> +"	.globl		my_tramp\n"
>  "   my_tramp:"
>  "	pushq %rbp\n"
>  "	movq %rsp, %rbp\n"
> diff --git a/samples/ftrace/ftrace-direct.c b/samples/ftrace/ftrace-direct.c
> index 63ca06d42c80..a2729d1ef17f 100644
> --- a/samples/ftrace/ftrace-direct.c
> +++ b/samples/ftrace/ftrace-direct.c
> @@ -14,6 +14,7 @@ extern void my_tramp(void *);
>  asm (
>  "	.pushsection    .text, \"ax\", @progbits\n"
>  "	.type		my_tramp, @function\n"
> +"	.globl		my_tramp\n"
>  "   my_tramp:"
>  "	pushq %rbp\n"
>  "	movq %rsp, %rbp\n"
> 
> base-commit: 585e5b17b92dead8a3aca4e3c9876fbca5f7e0ba