linux-kernel - Re: [PATCH] samples/ftrace: mark my

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Date:   Mon, 16 Nov 2020 13:27:46 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Colin Ian King <colin.king@...onical.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] samples/ftrace: mark my_tramp[12]? global

On Mon, Nov 16, 2020 at 1:18 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Mon, 16 Nov 2020 13:10:57 -0800
> Sami Tolvanen <samitolvanen@...gle.com> wrote:
>
> > Clang doesn't warn about this as we're building a module, it just
> > generates a reference to a non-existing global "my_tramp" symbol,
> > because the one defined in inline assembly has a local binding:
> >
> > $ readelf --symbols --wide ftrace-direct.lto.o | grep my_tramp
> >     16: 0000000000000000    13 FUNC    LOCAL  DEFAULT    1 my_tramp
> >     33: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND my_tramp
> >     42: 0000000000000000     8 FUNC    GLOBAL HIDDEN     8 my_tramp.cfi_jt
> >
> > This would prevent the module from loading, which modpost catches:
> >
> > ERROR: modpost: "my_tramp" [samples/ftrace/ftrace-direct.ko] undefined!
> >
> > > From user space, I'm just using the following file:
> >
> > As this error happens only with Control-Flow Integrity, we need to
> > take the address of the "test" function to force the compiler to
> > generate a jump table entry for it. Here's a slightly tweaked
> > stand-alone reproducer:
> >
> > https://godbolt.org/z/GnzjE4
>
> Thanks, we don't need to look more into this. It was mostly my curiosity to
> find a way to have the compiler know about a function declared statically
> in inline assembly. Maybe I'm asking for too much ;-)
>
> I'll take your original patch. Does it need to go to stable, or is this not
> that big of an issue to allow it to be added in the next merge window?

Thanks! This is definitely not a big issue, it just fixes the
allyesconfig build with CFI, and it's not needed in -stable.

Sami