Messages in this thread | | | From | Stephane Eranian <> | Date | Wed, 22 Jun 2022 19:51:02 +0200 | Subject | Re: [RFC] pr_warn_once() issue in x86 MSR extable code |
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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)) { \ > __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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