Re: [PATCH v4 7/7] rust: alloc: add Vec::insert_within_capacity

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Apr 30, 2025 at 01:39:03PM +0200, Greg KH wrote:
> On Wed, Apr 30, 2025 at 11:24:23AM +0000, Alice Ryhl wrote:
> > On Tue, Apr 29, 2025 at 05:30:06PM +0200, Greg KH wrote:
> > > On Tue, Apr 29, 2025 at 02:44:27PM +0000, Alice Ryhl wrote:
> > > > This adds a variant of Vec::insert that does not allocate memory. This
> > > > makes it safe to use this function while holding a spinlock. Rust Binder
> > > > uses it for the range allocator fast path.
> > > > 
> > > > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> > > > ---
> > > >  rust/kernel/alloc/kvec.rs | 39 +++++++++++++++++++++++++++++++++++++++
> > > >  1 file changed, 39 insertions(+)
> > > > 
> > > > diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> > > > index 0682108951675cbee05faa130e5a9ce72fc343ba..998afdcde47bec94b2c9d990ba3afbb3488ea99e 100644
> > > > --- a/rust/kernel/alloc/kvec.rs
> > > > +++ b/rust/kernel/alloc/kvec.rs
> > > > @@ -355,6 +355,45 @@ pub unsafe fn push_within_capacity_unchecked(&mut self, v: T) {
> > > >          unsafe { self.inc_len(1) };
> > > >      }
> > > >  
> > > > +    /// Inserts an element at the given index in the [`Vec`] instance.
> > > > +    ///
> > > > +    /// Fails if the vector does not have capacity for the new element. Panics if the index is out
> > > > +    /// of bounds.
> > > 
> > > Why panic and why not just return an error instead?
> > 
> > It's for consistency with stdlib. Illegal use is panic, expected error
> > conditions are errors.
> 
> But this is the kernel, not userspace :)
> 
> As you can return an error, why not?  Rebooting a box should be a "last
> resort" type of thing when you can not recover from an error.  You can
> easily not overflow and return an error here, so why do you want to just
> give up and cause all data to be lost?
> 
> And I don't see any other panics happening in this file, so would this
> be the first one?

I don't feel strongly about this method, but it's not the first panic.
The vector type has an indexing operator vec[i] that panics if you index
out-of-bounds.

Alice