Re: [PATCH 4/5] sched/core: Add __might_sleep_precision()

[Boqun Feng]

On Fri, May 09, 2025 at 08:00:32AM +0200, Ingo Molnar wrote:
> 
> * Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
> 
> > From: FUJITA Tomonori <fujita.tomonori@xxxxxxxxx>
> > 
> > Add __might_sleep_precision(), Rust friendly version of
> > __might_sleep(), which takes a pointer to a string with the length
> > instead of a null-terminated string.
> > 
> > Rust's core::panic::Location::file(), which gives the file name of a
> > caller, doesn't provide a null-terminated
> > string. __might_sleep_precision() uses a precision specifier in the
> > printk format, which specifies the length of a string; a string
> > doesn't need to be a null-terminated.
> > 
> > Modify __might_sleep() to call __might_sleep_precision() but the
> > impact should be negligible. When printing the error (sleeping
> > function called from invalid context), the precision string format is
> > used instead of the simple string format; the precision specifies the
> > the maximum length of the displayed string.
> > 
> > Note that Location::file() providing a null-terminated string for
> > better C interoperability is under discussion [1].
> > 
> > [1]: https://github.com/rust-lang/libs-team/issues/466
> > 
> > Tested-by: Daniel Almeida <daniel.almeida@xxxxxxxxxxxxx>
> > Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> > Co-developed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> > Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> > Signed-off-by: FUJITA Tomonori <fujita.tomonori@xxxxxxxxx>
> > Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> > Link: https://lore.kernel.org/r/20250410225623.152616-2-fujita.tomonori@xxxxxxxxx
> > ---
> >  include/linux/kernel.h |  2 ++
> >  kernel/sched/core.c    | 62 ++++++++++++++++++++++++++++--------------
> >  2 files changed, 43 insertions(+), 21 deletions(-)
> > 
> > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > index be2e8c0a187e..086ee1dc447e 100644
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -87,6 +87,7 @@ extern int dynamic_might_resched(void);
> >  #ifdef CONFIG_DEBUG_ATOMIC_SLEEP
> >  extern void __might_resched(const char *file, int line, unsigned int offsets);
> >  extern void __might_sleep(const char *file, int line);
> > +extern void __might_sleep_precision(const char *file, int len, int line);
> 
> Ugh.
> 
> Firstly, '_precision' is really ambiguous in this context and suggests 
> 'precise sleep' or something like that, which this is not about at all. 
> So the naming here is all sorts of bad already.
> 

I accept this is not a good naming.

> But more importantly, this is really a Rust problem. Does Rust really 
> have no NUL-terminated strings? It should hide them in shame and 

You can create NUL-terminated strings in Rust of course, but in this
case, because we want to use the "#[trace_caller]" attribute [1], which
allows might_sleep() in Rust to be defined as a function, and can use
Location::caller() to get the caller file and line number information,
and `Location` type yet doesn't return a Nul-terminated string literal,
so we have to work this around.

> construct proper, robust strings, instead of spreading this disease to 
> the rest of the kernel, IMHO ...
> 
> Rust is supposed to be about increased security, right? How does extra, 
> nonsensical complexity for simple concepts such as strings achieve 
> that? If the Rust runtime wants to hook into debug facilities of the 
> Linux kernel then I have bad news: almost all strings used by kernel 
> debugging facilities are NUL-terminated.

This is more of a special case because `Location` is used (i.e. file
name is the string literal). For things like user-defined string, we use
the macro c_str!(), which generates NUL-terminated strings. For example,
lockdep class names.

> 
> So I really don't like this patch. Is there no other way to do this?
> 

There's a `c_str` [2] macro which could generates a NUL-terminated
string, but using that will requires might_sleep() defined as a macro as
well. Given that might_sleep() is the user interface that most users
will use, and how it handles string literal for file names is an
implementation detail, so I figured it's better we resolve in the
current way.

[1]: https://rustc-dev-guide.rust-lang.org/backend/implicit-caller-location.html
[2]: https://rust.docs.kernel.org/kernel/macro.c_str.html

Regards,
Boqun

> Thanks,
> 
> 	Ingo