Subject: Re: [PATCHv2 1/8] ftrace: add ftrace_init_nop()

From: Mark Rutland <[email-masked]>
Date: Mon, 4 Nov 2019 13:36:58 +0000

On Sat, Nov 02, 2019 at 05:49:00PM +0530, Amit Daniel Kachhap wrote:
> Hi Mark,

Hi Amit,

Thanks for the review!
 
> On 10/29/19 10:28 PM, Mark Rutland wrote:
> > Architectures may need to perform special initialization of ftrace
> > callsites, and today they do so by special-casing ftrace_make_nop() when
> > the expected branch address is MCOUNT_ADDR. In some cases (e.g. for
> > patchable-function-entry), we don't have an mcount-like symbol and don't
> s/an mcount/a mcount.
> > want a synthetic MCOUNT_ADDR, but we may need to perform some
> > initialization of callsites.
> > 
> > To make it possible to separate initialization from runtime
> > modification, and to handle cases without an mcount-like symbol, this
> Same as above.

Using 'an' in both of these cases is correct, even though 'mcount'
starts with a consonant, since it's pronounced as-if it were 'emcount'.
I will leave this as-is.

> > patch adds an optional ftrace_init_nop() function that architectures can
> > implement, which does not pass a branch address.
> > 
> > Where an architecture does not provide ftrace_init_nop(), we will fall
> > back to the existing behaviour of calling ftrace_make_nop() with
> > MCOUNT_ADDR.
> > 
> > At the same time, ftrace_code_disable() is renamed to
> > ftrace_nop_initialize() to make it clearer that it is intended to
> > intialize a callsite into a disabled state, and is not for disabling a
> > callsite that has been runtime enabled. The kerneldoc description of rec
> > arguments is updated to cover non-mcount callsites.
> > 
> > Signed-off-by: Mark Rutland <[email-masked]>
> > Reviewed-by: Ard Biesheuvel <[email-masked]>
> > Cc: Ingo Molnar <[email-masked]>
> > Cc: Steven Rostedt <[email-masked]>
> > Cc: Torsten Duwe <[email-masked]>
> > ---
> >   include/linux/ftrace.h | 35 ++++++++++++++++++++++++++++++++---
> >   kernel/trace/ftrace.c  |  6 +++---
> >   2 files changed, 35 insertions(+), 6 deletions(-)
> > 
> > diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> > index 8a8cb3c401b2..9867d90d635e 100644
> > --- a/include/linux/ftrace.h
> > +++ b/include/linux/ftrace.h
> > @@ -499,7 +499,7 @@ static inline int ftrace_disable_ftrace_graph_caller(void) { return 0; }
> >   /**
> >    * ftrace_make_nop - convert code into nop
> >    * @mod: module structure if called by module load initialization
> > - * @rec: the mcount call site record
> > + * @rec: the call site record (e.g. mcount/fentry)
> >    * @addr: the address that the call site should be calling
> >    *
> >    * This is a very sensitive operation and great care needs
> > @@ -520,9 +520,38 @@ static inline int ftrace_disable_ftrace_graph_caller(void) { return 0; }
> >   extern int ftrace_make_nop(struct module *mod,
> >   			   struct dyn_ftrace *rec, unsigned long addr);
> > +
> > +/**
> > + * ftrace_init_nop - initialize a nop call site
> > + * @mod: module structure if called by module load initialization
> > + * @rec: the call site record (e.g. mcount/fentry)
> > + *
> > + * This is a very sensitive operation and great care needs
> > + * to be taken by the arch.  The operation should carefully
> > + * read the location, check to see if what is read is indeed
> > + * what we expect it to be, and then on success of the compare,
> > + * it should write to the location.
> > + *
> > + * The code segment at @rec->ip should contain the contents created by
> > + * the compiler
> Nit: Will it be better to write it as "@rec->ip should store the adjusted
> ftrace entry address of the call site" or something like that.

This was the specific wording requested by Steve, and it's trying to
describe the instructions at rec->ip, rather than the value of rec->ip,
so I think it's better to leave this as-is.

> > + *
> > + * Return must be:
> > + *  0 on success
> > + *  -EFAULT on error reading the location
> > + *  -EINVAL on a failed compare of the contents
> > + *  -EPERM  on error writing to the location
> > + * Any other value will be considered a failure.
> > + */
> > +#ifndef ftrace_init_nop
> > +static inline int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
> > +{
> > +	return ftrace_make_nop(mod, rec, MCOUNT_ADDR);
> > +}
> > +#endif
> > +
> Now that ftrace_init_nop is also an arch implemented function so it may be
> added in Documentation/trace/ftrace-design.rst along with ftrace_make_nop.
> In general also, adding some description about patchable-function-entry
> in kernel Documentation will be useful.

I agree that would be a good thing, but as Steve suggests I will leave
this for subsequent rework, as I think that also implies more
rework/renaming in the core code (e.g. to more cleanly separate the
notion of a callsite from mcount specifically).

Steve, if you'd like help with that (or review), I'd be happy to help.

Thanks,
Mark.

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