On Thu, Jun 18, 2020 at 01:37:46PM -0700, Matt Helsley wrote:
> recordmcount has its own ELF wrapper code and could utilize
> objtool's ELF code to more-portably handle architecture variations.
> This series makes recordmcount a subcommand of objtool. It very
> gradually converts recordmcount to become a subcommand of objtool and
> then reuses parts of objtool's ELF code.
>
> recordmcount maps the file in and collects simple information it needs to
> append a section to the object file. The only part of the original file it
> modifies is the address of new section tables -- interestingly enough this
> resembles RCU in that we don't really trim the old tables so
> much as unlink them via a critical offset and then rely on
> future tooling, in this case, to drop the unused bits.
>
> Much of the recordmcount ELF code is only reading and walking the data
> structures to collect the mcount locations it records in a separate
> area of memory. This means it's safe to mix access to the mapped
> file with access to the objtool-style linked data
> structures as we gradually convert it to using only the linked data
> structures. Once the old ELF code is no longer in use we can drop it
> and use objtool to take over the task of writing the results without
> using the RCU-like trick any more.
>
> After that we greatly simplify the mcount subcommand by adding a
> few flags to the ELF reading code in objtool. Overall the series
> removes about 600 lines of recordmcount while adding little to
> objtool's ELF code.
>
> Testing so far:
>
> I've been using scripts to test cross compilation and execution of
> objtool, and mcount on objects built for x86, ppc64le, arm64, s390, and
> sparc.
>
>
> Applies on top of:
> objtool/core
>
> Peter Zijlstra's "x86/entry: noinstr fixes" [2]
Just thought I'd note for anyone reviewing/playing with this series:
Peter's patches are now in objtool/core
> Sami Tolvanen's patch enabling support for more than 64k
> sections in recordmcount, already going upstream. [3]
Sami's patch is now in Linus' master branch
Cheers,
-Matt
