Re: [PATCH 1/2] strbuf: use st_add3() in strbuf_grow()

[Jeff King <peff@peff.net>]

On Fri, May 15, 2026 at 04:30:34PM +0200, René Scharfe wrote:

> > That's all assuming that no overflow happens before ALLOC_GROW() gets
> > the values. We also tend to do unchecked computions for the "nr" field
> > there, but it's usually just "nr_foo + 1", so the same logic applies:
> > you'd have to have an existing array consuming the entire address space
> > minus one byte to trigger an overflow.
>
> The use in read-cache.c::do_read_index() looks odd.  Has been present
> since commit one.  Is the point that it over-allocates to have room for
> additions right from the start?  For read-only commands this only wastes
> memory, no?

Hmm, yeah, that is weird, and unusual to use alloc_nr() directly. We are
presumably picking up istate->cache_nr from the on-disk file, so it
could be anything, and that alloc_nr() could overflow.

We'd store the too-small value in alloc, so we _know_ it's too small. So
later when we use ALLOC_GROW(), the problem would be resolved as we grow
the array. I'm not convinced the initial load might not overflow the
array, though.

-Peff