Messages in this thread Patch in this message | | | Date | Fri, 17 Jun 2022 16:52:06 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC] pr_warn_once() issue in x86 MSR extable code |
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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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