* Martin Sebor msebor@gmail.com wrote:
I.e. the real workaround might be to turn off the -Wstringop-overread-warning, until GCC-11 gets fixed?
In GCC 10 -Wstringop-overread is a subset of -Wstringop-overflow. GCC 11 breaks it out as a separate warning to make it easier to control. Both warnings have caught some real bugs but they both have a nonzero rate of false positives. Other than bug reports we don't have enough data to say what their S/N ratio might be but my sense is that it's fairly high in general.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=wstringop-overread https://gcc.gnu.org/bugzilla/show_bug.cgi?id=wstringop-overflow
In GCC 11, all access warnings expect objects to be either declared or allocated. Pointers with constant values are taken to point to nothing valid (as Arnd mentioned above, this is to detect invalid accesses to members of structs at address zero).
One possible solution to the known address problem is to extend GCC attributes address and io that pin an object to a hardwired address to all targets (at the moment they're supported on just one or two targets). I'm not sure this can still happen before GCC 11 releases sometime in April or May.
Until then, another workaround is to convert the fixed address to a volatile pointer before using it for the access, along the lines below. It should have only a negligible effect on efficiency.
Thank you for the detailed answer!
I think I'll go with Arnd's original patch - which makes the code a slightly bit cleaner by separating out the check_tboot_version() check into a standalone function.
The only ugly aspect is the global nature of the 'tboot' pointer - but that's a self-inflicted wound.
I'd also guess that the S/N ratio somewhat unfairly penalizes this warning right now, because the kernel had a decade of growing real fixes via other efforts such as static and dynamic instrumentation as well.
So the probability of false positive remaining is in fact higher, and going forward we should see a better S/N ratio of this warning. Most of which will never be seen by upstream maintainers, as the mishaps will stay at the individual developer level. :-)
Thanks,
Ingo