On Fri, Sep 13, 2024 at 5:23 PM Thorsten Blum <[email protected]> wrote: > > 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.
The 4294967295 simply means "I don't know." There's probably a bug in the size calculation. I'll look into it. -bw
