On Apr 8, 2016 9:20 AM, "Dmitry Safonov" <dsafonov@xxxxxxxxxxxxx> wrote: > > On 04/08/2016 06:56 PM, Andy Lutomirski wrote: >> >> On Fri, Apr 8, 2016 at 6:50 AM, Dmitry Safonov <dsafonov@xxxxxxxxxxxxx> wrote: >>> >>> Hello again, >>> what do you think about attached patch? >>> I think it should fix landing problem for i386 vdso mremap. >>> It does not touch fast syscall path, so there should be no >>> speed regression. >> >> For this thing: >> >> >> + /* Fixing userspace landing - look at do_fast_syscall_32 */ >> + if (current_thread_info()->status & TS_COMPAT) >> + regs->ip = (unsigned long)current->mm->context.vdso + >> + vdso_image_32.sym_int80_landing_pad; >> >> Either check that ip was where you expected it > > And if it's not there - return error? No, just leave IP unchanged. > >> or simply remove this >> code -- user programs that are mremapping the vdso are already playing >> with fire and can just use int $0x80 to do it. >> >> Other than that, it looks generally sane. The .mremap hook didn't >> exist last time I looked at this :) >> >> The main downside of your approach is that it doesn't allow switching >> between the 32-bit, 64-bit, and x32 images. Also, it requires >> awareness of how vvar and vdso line up, whereas a dedicated API could >> do the whole thing. > > Yes, I'm working on it. This patch will only allow moving vdso > image with general mremap - so I could use arch_prctl for > that API, as for native i386 one may move vdso with mremap > and cannot map any other vdso blobs. > Does it sound fine? > > So, I have some difficulties with removing TIF_IA32 flag: > it's checked by perf for interpreting stack frames/instructions > and may be checked out of syscall executing (when tracing > page fault events, for example). Feel free to ask for help on some of these details. user_64bit_mode will be helpful too. > I doubt, is it sane to remove > TS_COMPAT instead, leaving TIF_IA32, as for some cases > we need to know if task is compatible outside of syscall's path? No. TS_COMPAT is important, and it's also better behaved than TIF_IA32 -- it has a very specific meaning: "am I currently executing a 32-bit syscall". > And the comment in asm/syscall.h says: > > * TIF_IA32 tasks should always have TS_COMPAT set at > > * system call time. > that means, that TS_COMPAT is always set on TIF_IA32, so > is meaningless. > What do you think? The comment is wrong :). TS_COMPAT is true on int80 or 32-bit vdso syscall entries and is false otherwise. 64-bit tasks can use int80 and, with your patches, will be able to use the 32-bit vdso entry as well. > > Thanks, > Dmitry. -- To unsubscribe from this list: send the line "unsubscribe linux-kselftest" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
