Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()

[Josh Poimboeuf]

On Thu, Oct 12, 2023 at 07:59:43PM +0200, Ingo Molnar wrote:
> 
> * Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> 
> > On Thu, Oct 12, 2023 at 08:19:14AM +0200, Ingo Molnar wrote:
> > > 
> > > * Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > > 
> > > > Though, another problem is that .text has a crazy amount of padding
> > > > which makes it always the same size, due to the SRSO alias mitigation
> > > > alignment linker magic.  We should fix that somehow.
> > > 
> > > We could emit a non-aligned end-of-text symbol (we might have it already),
> > > and have a script or small .c program in scripts/ or tools/ that looks
> > > at vmlinux and displays a user-friendly and accurate list of text and
> > > data sizes in the kernel?
> > > 
> > > And since objtool is technically an 'object files tool', and it already
> > > looks at sections & symbols, it could also grow a:
> > > 
> > > 	objtool size <objfile>
> > > 
> > > command that does the sane thing ... I'd definitely start using that, instead of 'size'.
> > > 
> > > /me runs :-)
> > 
> > Yeah, that's actually not a bad idea.
> > 
> > I had been thinking a "simple" script would be fine, but I'm realizing
> > the scope of this thing could grow over time.  In which case a script is
> > less than ideal.  And objtool already has the ability to do this pretty
> > easily.
> 
> Yeah, and speed actually matters here: I have scripts that generate object 
> comparisons between commits, and every second of runtime counts - and a 
> script would be slower and more fragile for something like allmodconfig 
> builds or larger disto configs.

Ah, good to know.

> BTW., maybe the right objtool subcommand would be 'objtool sections', with 
> an 'objtool sections size' sub-sub-command. Because I think this discussion 
> shows that it would be good to have a bit of visibility into the sanity of 
> our sections setup, with 'objtool sections check' for example doing a 
> sanity check on whether there's anything extra in the text section that 
> shouldn't be there? Or so ...

What would be an example of something "extra"?  A sanity check might fit
better alongside the other checks already being done by the main objtool
"subcommand" which gets run by the kernel build.

BTW, I actually removed subcommands a while ago when I overhauled
objtool's interface to make it easier to combine options.  That said,
I'm not opposed to re-adding them if we can find a sane way to do so.

Here's the current interface:

 Usage: objtool <actions> [<options>] file.o

Actions:
    -h, --hacks[=<jump_label,noinstr,skylake>]
                          patch toolchain bugs/limitations
    -i, --ibt             validate and annotate IBT
    -l, --sls             validate straight-line-speculation mitigations
    -m, --mcount          annotate mcount/fentry calls for ftrace
    -n, --noinstr         validate noinstr rules
    -o, --orc             generate ORC metadata
    -r, --retpoline       validate and annotate retpoline usage
    -s, --stackval        validate frame pointer rules
    -t, --static-call     annotate static calls
    -u, --uaccess         validate uaccess rules for SMAP
        --cfi             annotate kernel control flow integrity (kCFI) function preambles
        --dump[=<orc>]    dump metadata
        --prefix <n>      generate prefix symbols
        --rethunk         validate and annotate rethunk usage
        --unret           validate entry unret placement

Options:
    -v, --verbose         verbose warnings
        --backtrace       unwind on error
        --backup          create .orig files before modification
        --dry-run         don't write modifications
        --link            object is a linked object
        --mnop            nop out mcount call sites
        --module          object is part of a kernel module
        --no-unreachable  skip 'unreachable instruction' warnings
        --sec-address     print section addresses in warnings
        --stats           print statistics


Note how all the actions can be easily combined in a single execution
instance.

If we re-added subcommands, most of the existing functionality would be
part of a single subcommand.  It used to be called "check", but it's no
longer a read-only operation so that's misleading.  I'll call it "run"
for now.

Right now my preference would be to leave the existing interface as-is,
and then graft optional subcommands on top.  If no subcommand is
specified then it would default to the "run" subcommand.  It's a little
funky, but it would work well for the common case, where ~99% of the
functionality lives.  And it doesn't break existing setups and
backports.

For example:

  # current interface (no changes)
  objtool --mcount --orc --retpoline --uaccess vmlinux.o

  # same, with optional explicit "run" subcommand
  objtool run --mcount --orc --retpoline --uaccess vmlinux.o

  # new "size" subcommand
  obtool size [options] vmlinux.o.before vmlinux.o.after

-- 
Josh