On Mon, Oct 07, 2019 at 04:44:23PM +0200, Ingo Molnar wrote: > > * Ingo Molnar <mi...@kernel.org> wrote: > > > > All the other reasons would require a fairly egregious kernel bug, hence > > > the speculation that the #GP is due to a non-canonical address. Something > > > like the following would be more precise, though highly unlikely to ever > > > be exercised, e.g. KVM had a fatal bug related to injecting a non-zero > > > error code that went unnoticed for years. > > > > > > WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault in user > > > access. %s?\n", > > > (IS_ENABLED(CONFIG_X86_64) && !error_code) ? "Non-canonical > > > address" : > > > "Segmentation > > > bug"); > > > > Instead of trying to guess the reason of the #GPF (which guess might be > > wrong), please just state it as the reason if we are sure that the cause > > is a non-canonical address - and provide a best-guess if it's not but > > clearly signal that it's a guess. > > > > I.e. if I understood all the cases correctly we'd have three types of > > messages generated: > > > > !error_code: > > "General protection fault in user access, due to non-canonical address."
A non-canonical #GP always has an error code of '0', but the reverse isn't technically true. And 32-bit mode obviously can't generate a non-canonical address. But practically speaking, since _ASM_EXTABLE_UA() should only be used for reg<->mem instructions, the only way to get a #GP on a usercopy instruction would be to corrupt the code itself or have a bad segment loaded in 32-bit mode. So qualifying the non-canonical message on '64-bit && !error_code' is techncally more precise/correct, but likely meaningless in practice. > > error_code && !is_canonical_addr(fault_addr): > > "General protection fault in user access. Non-canonical address?" > > > > error_code && is_canonical_addr(fault_addr): > > "General protection fault in user access. Segmentation bug?" > > Now that I've read the rest of the thread, since fault_addr is always 0 > we can ignore most of this I suspect ...
