linux-kernel - Re: [RFC][PATCH v5 00/51] objtool: Make recordmcount a subcommand

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Date:   Tue, 23 Jun 2020 07:12:19 -0700
From:   Matt Helsley <mhelsley@...are.com>
To:     <linux-kernel@...r.kernel.org>
CC:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Julien Thierry <jthierry@...hat.com>,
        Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
Subject: Re: [RFC][PATCH v5 00/51] objtool: Make recordmcount a subcommand

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