Re: [PATCH] x86/ftrace: Get rid of function_hook

[Steven Rostedt]

On Mon, 21 Oct 2019 16:10:38 +0200
Borislav Petkov <bp@xxxxxxxxx> wrote:

> From: Borislav Petkov <bp@xxxxxxx>
> 
> function_hook is used as a better name than the default __fentry__
> which is the profiling counter which gcc adds before every function's
> prologue. Thus, it is not called from C and cannot have the same
> semantics as a pure C function - it saves/restores regs so that a C
> function can be called.
> 
> Drop the function_hook symbol and use __fentry__ directly for better
> alignment with gcc's documentation.
> 
> Switch the marking to SYM_CODE_START/_END which is reserved for
> non-standard, special functions.

I would break this into two patches. One that removes the function_hook
name, and the other to convert to the SYM_CODE_START/END.

For the removal of the function_hook patch, be sure to state some of
the history for why the function_hook was created in the first place.
Which would be something like this:

"When ftrace first was introduced to the kernel, it used gcc's
mcount profiling mechanism. The mcount mechanism would add a call to
"mcount" at the start of every function but after the stack frame was
set up. Later, in gcc 4.6, gcc introduced -mfentry, that would create a
call to "__fentry__" instead of "mcount", before the stack frame was
set up. In order to handle both cases, ftrace defined a macro
"function_hook" that would be either "mcount" or "__fentry__" depending
on which one was being used.

The Linux kernel no longer supports the "mcount" method, thus there's
no reason to keep the "function_hook" define around. Simply use
"__fentry__", as there is no ambiguity to the name anymore."

-- Steve


> 
> Signed-off-by: Borislav Petkov <bp@xxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Jiri Slaby <jslaby@xxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Cc: linux-doc@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> ---
>  Documentation/asm-annotations.rst |  4 ++--
>  arch/x86/kernel/ftrace_32.S       |  8 +++-----
>  arch/x86/kernel/ftrace_64.S       | 13 ++++++-------
>  3 files changed, 11 insertions(+), 14 deletions(-)
>