Messages in this thread | | | From | Stephane Eranian <> | Date | Wed, 20 Jul 2022 15:48:56 +0300 | Subject | Re: [RFC] pr_warn_once() issue in x86 MSR extable code |
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On Wed, Jun 22, 2022 at 8:51 PM Stephane Eranian <eranian@google.com> wrote: > > Hi Peter, > > Thanks for taking a quick look at this. > I am currently OOO and I cannot test this proposed patch. > I am okay with your suggestion. > > Thanks. > > On Fri, Jun 17, 2022 at 4:52 PM Peter Zijlstra <peterz@infradead.org> wrote: > > > > On Fri, Jun 17, 2022 at 02:08:52PM +0300, Stephane Eranian wrote: > > > Hi, > > > > > > Some changes to the way invalid MSR accesses are reported by the kernel is > > > causing some problems with messages printed on the console. > > > > > > We have seen several cases of ex_handler_msr() printing invalid MSR > > > accesses once but > > > the callstack multiple times causing confusion on the console. > > > > > > The last time the exception MSR code was modified (5.16) by PeterZ was: > > > > > > d52a7344bdfa x86/msr: Remove .fixup usage: > > > > > > if (!safe && wrmsr && pr_warn_once("unchecked MSR access error: ...")) > > > show_stack_regs(regs); > > > > > > Note that this code pattern was also present, though in a different > > > form, before this commit. > > > > > > The problem here is that another earlier commit (5.13): > > > > > > a358f40600b3 once: implement DO_ONCE_LITE for non-fast-path "do once" > > > functionality > > > > > > Modifies all the pr_*_once() calls to always return true claiming that > > > no caller is ever > > > checking the return value of the functions. > > > > > > This is why we are seeing the callstack printed without the associated > > > printk() msg. > > > > > > I believe that having the pr_*_once() functions return true the first > > > time they are called > > > is useful especially when extra information, such as callstack, must > > > be printed to help > > > track the origin of the problem. > > > > > > The exception handling code seems to be the only place where the > > > return value is checked > > > for pr_warn_once(). A minimal change would be to create another > > > version of that function > > > that calls DO_ONCE() instead of DO_ONCE_LITE(), e.g., pr_warn_once_return(). > > > > > > I can post a patch to that effect if we all agree on the approach. > > > > > > Thanks. > > > > How about something like this? > > > > diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c > > index dba2197c05c3..331310c29349 100644 > > --- a/arch/x86/mm/extable.c > > +++ b/arch/x86/mm/extable.c > > @@ -94,16 +94,18 @@ static bool ex_handler_copy(const struct exception_table_entry *fixup, > > static bool ex_handler_msr(const struct exception_table_entry *fixup, > > struct pt_regs *regs, bool wrmsr, bool safe, int reg) > > { > > - if (!safe && wrmsr && > > - pr_warn_once("unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x) at rIP: 0x%lx (%pS)\n", > > - (unsigned int)regs->cx, (unsigned int)regs->dx, > > - (unsigned int)regs->ax, regs->ip, (void *)regs->ip)) > > + if (__ONCE_LITE_IF(!safe && wrmsr)) { > > + pr_warn("unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x) at rIP: 0x%lx (%pS)\n", > > + (unsigned int)regs->cx, (unsigned int)regs->dx, > > + (unsigned int)regs->ax, regs->ip, (void *)regs->ip); > > show_stack_regs(regs); > > + } > > > > - if (!safe && !wrmsr && > > - pr_warn_once("unchecked MSR access error: RDMSR from 0x%x at rIP: 0x%lx (%pS)\n", > > - (unsigned int)regs->cx, regs->ip, (void *)regs->ip)) > > + if (__ONCE_LITE_IF(!safe && !wrmsr)) { > > + pr_warn("unchecked MSR access error: RDMSR from 0x%x at rIP: 0x%lx (%pS)\n", > > + (unsigned int)regs->cx, regs->ip, (void *)regs->ip); > > show_stack_regs(regs); > > + } > > > > if (!wrmsr) { > > /* Pretend that the read succeeded and returned 0. */ > > diff --git a/include/linux/once_lite.h b/include/linux/once_lite.h > > index 861e606b820f..63c3bbcef694 100644 > > --- a/include/linux/once_lite.h > > +++ b/include/linux/once_lite.h > > @@ -9,15 +9,27 @@ > > */ > > #define DO_ONCE_LITE(func, ...) \ > > DO_ONCE_LITE_IF(true, func, ##__VA_ARGS__) > > -#define DO_ONCE_LITE_IF(condition, func, ...) \ > > + > > +#define __ONCE_LITE_IF(condition) \ > > ({ \ > > static bool __section(".data.once") __already_done; \ > > - bool __ret_do_once = !!(condition); \ > > + bool __ret_cond = !!(condition); \ > > + bool __ret_once = false; \ > > \ > > if (unlikely(__ret_do_once && !__already_done)) { \
You need to replace __ret_do_once with __ret_cond above and then it compiles and works. I have tested with a kernel module that reads and writes to an illegal MSR:
unchecked MSR access error: RDMSR from 0x1234567 at rIP: 0xffffffffc00ec138 (rdpmc_intel+0x28/0x21d0 [rdpmc_test]) Call Trace: <TASK> rdpmc_bench_store+0x53/0x80 [rdpmc_test] kobj_attr_store+0xf/0x20 sysfs_kf_write+0x34/0x50 kernfs_fop_write_iter+0xfa/0x180 vfs_write+0x334/0x3d0 ksys_write+0x71/0xe0 __x64_sys_write+0x1b/0x20 do_syscall_64+0x44/0xa0 ? exc_page_fault+0x6e/0x110 entry_SYSCALL_64_after_hwframe+0x63/0xcd
Tested-by: Stephane Eranian <eranian@google.com>
> > > __already_done = true; \ > > - func(__VA_ARGS__); \ > > + __ret_once = true; \ > > } \ > > + unlikely(__ret_once); \ > > + }) > > + > > +#define DO_ONCE_LITE_IF(condition, func, ...) \ > > + ({ \ > > + bool __ret_do_once = !!(condition); \ > > + \ > > + if (__ONCE_LITE_IF(__ret_do_once)) \ > > + func(__VA_ARGS__); \ > > + \ > > unlikely(__ret_do_once); \ > > }) > >
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