Re: [PATCH v3 21/27] x86/ftrace: Adapt function tracing for PIE support

Thomas Garnier via Virtualization
2018-05-24T20:41:27Z

On Thu, May 24, 2018 at 1:16 PM Steven Rostedt <rostedt@goodmis.org> wrote:

> On Thu, 24 May 2018 13:40:24 +0200
> Petr Mladek <pmladek@suse.com> wrote:

> > On Wed 2018-05-23 12:54:15, Thomas Garnier wrote:
> > > When using -fPIE/PIC with function tracing, the compiler generates a
> > > call through the GOT (call *__fentry__@GOTPCREL). This instruction
> > > takes 6 bytes instead of 5 on the usual relative call.
> > >
> > > If PIE is enabled, replace the 6th byte of the GOT call by a 1-byte
nop
> > > so ftrace can handle the previous 5-bytes as before.
> > >
> > > Position Independent Executable (PIE) support will allow to extended
the
> > > KASLR randomization range below the -2G memory limit.
> > >
> > > Signed-off-by: Thomas Garnier <thgarnie@google.com>
> > > ---
> > > arch/x86/include/asm/ftrace.h | 6 +++--
> > > arch/x86/include/asm/sections.h | 4 ++++
> > > arch/x86/kernel/ftrace.c | 42
+++++++++++++++++++++++++++++++--
> > > 3 files changed, 48 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/arch/x86/include/asm/ftrace.h
b/arch/x86/include/asm/ftrace.h
> > > index c18ed65287d5..8f2decce38d8 100644
> > > --- a/arch/x86/include/asm/ftrace.h
> > > +++ b/arch/x86/include/asm/ftrace.h
> > > @@ -25,9 +25,11 @@ extern void __fentry__(void);
> > > static inline unsigned long ftrace_call_adjust(unsigned long addr)
> > > {
> > > /*
> > > - * addr is the address of the mcount call instruction.
> > > - * recordmcount does the necessary offset calculation.
> > > + * addr is the address of the mcount call instruction. PIE has
always a
> > > + * byte added to the start of the function.
> > > */
> > > + if (IS_ENABLED(CONFIG_X86_PIE))
> > > + addr -= 1;
> >
> > This seems to modify the address even for modules that are _not_
compiled with
> > PIE, see below.

> Can one load a module not compiled for PIE in a kernel with PIE?

> >
> > > return addr;
> > > }
> > >
> > > diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> > > index 01ebcb6f263e..73b3c30cb7a3 100644
> > > --- a/arch/x86/kernel/ftrace.c
> > > +++ b/arch/x86/kernel/ftrace.c
> > > @@ -135,6 +135,44 @@ ftrace_modify_code_direct(unsigned long ip,
unsigned const char *old_code,
> > > return 0;
> > > }
> > >
> > > +/* Bytes before call GOT offset */
> > > +const unsigned char got_call_preinsn[] = { 0xff, 0x15 };
> > > +
> > > +static int
> > > +ftrace_modify_initial_code(unsigned long ip, unsigned const char
*old_code,
> > > + unsigned const char *new_code)
> > > +{
> > > + unsigned char replaced[MCOUNT_INSN_SIZE + 1];
> > > +
> > > + ftrace_expected = old_code;
> > > +
> > > + /*
> > > + * If PIE is not enabled or no GOT call was found, default to the
> > > + * original approach to code modification.
> > > + */
> > > + if (!IS_ENABLED(CONFIG_X86_PIE) ||
> > > + probe_kernel_read(replaced, (void *)ip, sizeof(replaced)) ||
> > > + memcmp(replaced, got_call_preinsn, sizeof(got_call_preinsn)))
> > > + return ftrace_modify_code_direct(ip, old_code, new_code);
> >
> > And this looks like an attempt to handle modules compiled without
> > PIE. Does it works with the right ip in that case?

> I'm guessing the || is for the "or no GOT call was found", but it
> doesn't explain why no GOT would be found.

Yes, maybe I could have made it work by using text_ip_addr earlier.

> >
> > I wonder if a better solution would be to update
> > scripts/recordmcount.c to store the incremented location into the
module.

I will look into it.

> If recordmcount.c can handle this, then I think that's the preferred
> approach. Thanks!

> -- Steve

> >
> > IMPORTANT: I have only vague picture about how this all works. It is
> > possible that I am completely wrong. The code might be correct,
> > especially if you tested this situation.
> >
> > Best Regards,
> > Petr

--
Thomas