RE: [PATCH v4 12/20] vsprintf: add new `%pA` format specifier

[David Laight]

From: Rasmus Villemoes
> Sent: 14 February 2022 10:53
> 
> On 14/02/2022 11.18, Andy Shevchenko wrote:
> > On Sat, Feb 12, 2022 at 02:03:38PM +0100, Miguel Ojeda wrote:
> >
> >> From: Gary Guo <gary@xxxxxxxxxxx>
> >
> > Not sure I understand this...
> >
> >> This patch adds a format specifier `%pA` to `vsprintf` which formats
> >> a pointer as `core::fmt::Arguments`. Doing so allows us to directly
> >> format to the internal buffer of `printf`, so we do not have to use
> >> a temporary buffer on the stack to pre-assemble the message on
> >> the Rust side.
> >>
> >> This specifier is intended only to be used from Rust and not for C, so
> >> `checkpatch.pl` is intentionally unchanged to catch any misuse.
> >>
> >> Co-developed-by: Alex Gaynor <alex.gaynor@xxxxxxxxx>
> >> Signed-off-by: Alex Gaynor <alex.gaynor@xxxxxxxxx>
> >> Co-developed-by: Wedson Almeida Filho <wedsonaf@xxxxxxxxxx>
> >> Signed-off-by: Wedson Almeida Filho <wedsonaf@xxxxxxxxxx>
> >
> >> Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>
> >
> > ...together with this in the current SoB chain.
> >
> >> Co-developed-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> >> Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> >
> > I'm wondering if you considered to use %pV.
> >
> 
> I think the point is for vsnprintf() to call (back) into Rust code.

Doesn't that stand a reasonable chance of blowing the kernel stack?

vsnprintf() is likely to be on the 'worst case' stack path anyway.
Anything vaguely like a recursive call, or anything 'stack expensive'
inside vsnprintf() stands a real chance of overflowing the stack.

	David

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