On 11/20/2018 03:19 PM, Joe Lawrence wrote:
> Hi Steve,
>
> I noticed that ftrace does not currently support functions built with
> the -ffunction-sections option as they end up in .text.<function_name>
> ELF sections, never making into the __mcount_loc section.
>
> I modified the recordmcount scripts to handle such .text. section
> prefixes and this appears to work on x86_64, ppc64le, and s390x -- at
> least for simple modules, including the "Test trace_printk from module"
> ftrace self-test.
>
> That said, I did notice 90ad4052e85c ("kbuild: avoid conflict between
> -ffunction-sections and -pg on gcc-4.7") which indicates that the kernel
> still supports versions of gcc which may not play well with ftrace and
> -ffunction-sections.
>
> With that limitation in mind, can we support ftracing of functions in
> such sections for compiler versions that do support it? (fwiw, gcc
> v4.8.5 seems happy) And then if so, what additional testing or coding
> would need to be done to be confident that it is safe? Is matching on
> ".text.*" too inclusive?
>
Gentle ping... I took a dive through the rhkl-archives and found a few
older discussions:
[PATCH] scripts/recordmcount.pl: Support build with -ffunction-sections.
https://lore.kernel.org/lkml/CAFbHwiRtBaHkpZqTm6VZ=fCJcyu+dsdpo_kxMHy1egce=rt...@mail.gmail.com/
and related LWN article:
The source of the e1000e corruption bug
https://lwn.net/Articles/304105/
Catching up with those, I assume that this has never been implemented in
the past due to fear of ftrace modifying a potentially freed section
(and bricking NICs in the process :(
Looking through the kernel sources (like Will in 2008) I don't see any
code jumping out at me that frees code other than .init. However a
quick code inspection is no guarantee.
Assuming the same use-after-free reservation holds true today:
1: Is there any reasonable way to mark code sections (pages?) as
in-use to avoid memory freeing mechanisms from releasing them? The
logic for .init is mostly arch-specific, so there could be many
different ways random arches may try to pull this off.
2: Would/could it be safer to restrict __mcount_loc detection of
".text.*" sections to modules? The recordmcount.pl script already
knows about is_module... that information could be provided to
recordmcount.c as well for consideration.
-- Joe
