On 14. Sep 2024, at 01:44, Nathan Chancellor <[email protected]> wrote: > On Sat, Sep 14, 2024 at 01:32:19AM +0200, Thorsten Blum wrote: >> Thanks for reporting this. >> >> Changing >> >> memset(&mk->mp->attrs[mk->mp->num - 1], 0, sizeof(mk->mp->attrs[0])); >> >> to >> >> memset(mk->mp->attrs + mk->mp->num - 1, 0, sizeof(mk->mp->attrs[0])); >> >> fixes the false-positive warning >> >> memset: detected buffer overflow: 32 byte write of buffer size 0 >> >> even though the pointers have the same value. Does anyone know why? > > Might be a good question for Bill? The full context is available > starting at: > > https://lore.kernel.org/all/20240913164630.GA4091534@thelio-3990X/ > > I wonder if the krealloc() has something to do with it? I should try GCC > but I don't have a tip of tree copy handy at the moment and I am also > rushing at the end of my day to pack for my travels to LPC :)
I think the problem is with __builtin_dynamic_object_size(). memset(p,,) calls __struct_size(p), which calls __builtin_dynamic_object_size(p, 0) and this behaves weirdly: __builtin_dynamic_object_size(&mk->mp->attrs[mk->mp->num - 1], 0); evaluates to 0, but __builtin_dynamic_object_size(mk->mp->attrs + mk->mp->num - 1, 0); evaluates to 4294967295. Both values are wrong, but the latter doesn't trigger the false-positive warning.
