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SubjectRe: [RFC] pr_warn_once() issue in x86 MSR extable code
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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