Hi Geert, On Wed, Dec 12, 2018 at 10:43:33AM +0100, Geert Uytterhoeven wrote: > On Wed, Dec 12, 2018 at 10:27 AM Dmitry V. Levin wrote: > > On Wed, Dec 12, 2018 at 10:01:29AM +0100, Geert Uytterhoeven wrote: > > > On Wed, Dec 12, 2018 at 9:55 AM Dmitry V. Levin wrote: > > > > On Mon, Dec 10, 2018 at 04:30:25PM +0300, Dmitry V. Levin wrote: > > > > > On Mon, Dec 10, 2018 at 02:06:28PM +0100, Geert Uytterhoeven wrote: > > > > > > On Mon, Dec 10, 2018 at 1:41 PM Dmitry V. Levin wrote: > > > > > > > On Mon, Dec 10, 2018 at 09:45:42AM +0100, Geert Uytterhoeven wrote: > > > > > > > > On Mon, Dec 10, 2018 at 5:30 AM Dmitry V. Levin wrote: > > > > > > > > > syscall_get_* functions are required to be implemented on all > > > > > > > > > architectures in order to extend the generic ptrace API with > > > > > > > > > PTRACE_GET_SYSCALL_INFO request. > > > > > > > > > > > > > > > > > > This introduces asm/syscall.h on m68k implementing all 5 syscall_get_* > > > > > > > > > functions as documented in asm-generic/syscall.h: syscall_get_nr, > > > > > > > > > syscall_get_arguments, syscall_get_error, syscall_get_return_value, > > > > > > > > > and syscall_get_arch. > > > > > > > > > > > > > > > > > > Cc: Geert Uytterhoeven > > > > > > > > > Cc: Oleg Nesterov > > > > > > > > > Cc: Andy Lutomirski > > > > > > > > > Cc: Elvira Khabirova > > > > > > > > > Cc: Eugene Syromyatnikov > > > > > > > > > Cc: linux-m68k@lists.linux-m68k.org > > > > > > > > > Signed-off-by: Dmitry V. Levin > > > > > > > > > --- > > > > > > > > > > > > > > > > > > Notes: > > > > > > > > > v5: added syscall_get_nr, syscall_get_arguments, syscall_get_error, > > > > > > > > > and syscall_get_return_value > > > > > > > > > v1: added syscall_get_arch > > > > > > > > > > > > > > > > > --- /dev/null > > > > > > > > > +++ b/arch/m68k/include/asm/syscall.h > > > > > > > > > @@ -0,0 +1,39 @@ > > > > > > > > > > > > > > > > > +static inline void > > > > > > > > > +syscall_get_arguments(struct task_struct *task, struct pt_regs *regs, > > > > > > > > > + unsigned int i, unsigned int n, unsigned long *args) > > > > > > > > > +{ > > > > > > > > > + BUG_ON(i + n > 6); > > > > > > > > > > > > > > > > Does this have to crash the kernel? > > > > > > > > > > > > > > This is what most of other architectures do, but we could choose > > > > > > > a softer approach, e.g. use WARN_ON_ONCE instead. > > > > > > > > > > > > > > > Perhaps you can return an error code instead? > > > > > > > > > > > > > > That would be problematic given the signature of this function > > > > > > > and the nature of the potential bug which would most likely be a usage error. > > > > > > > > > > > > Of course to handle that, the function's signature need to be changed. > > > > > > Changing it has the advantage that the error handling can be done at the > > > > > > caller, in common code, instead of duplicating it for all > > > > > > architectures, possibly > > > > > > leading to different semantics. > > > > > > > > > > Given that *all* current users of syscall_get_arguments specify i == 0 > > > > > (and there is an architecture that has BUG_ON(i)), > > > > > it should be really a usage error to get into situation where i + n > 6, > > > > > I wish a BUILD_BUG_ON could be used here instead. > > > > > > > > > > I don't think it worths pushing the change of API just to convert > > > > > a "cannot happen" assertion into an error that would have to be dealt with > > > > > on the caller side. > > > > > > > > I suggest the following BUG_ON replacement for syscall_get_arguments: > > > > > > > > #define SYSCALL_MAX_ARGS 6 > > > > > > > > static inline void > > > > syscall_get_arguments(struct task_struct *task, struct pt_regs *regs, > > > > unsigned int i, unsigned int n, unsigned long *args) > > > > { > > > > /* > > > > * Ideally there should have been > > > > * BUILD_BUG_ON(i + n > SYSCALL_MAX_ARGS); > > > > * instead of these checks. > > > > */ > > > > if (unlikely(i > SYSCALL_MAX_ARGS)) { > > > > WARN_ONCE(1, "i > SYSCALL_MAX_ARGS"); > > > > return; > > > > > > Does this have security implications, as args is an output parameter? > > > I.e. if you don't fill the array, the caller will use whatever is on the stack. > > > Can this ever be passed to userspace, leaking data? > > > > In the current kernel code n is always less or equal to 6, > > but in theory future changes can potentially break the assertion > > and this could lead to leaking data to userspace. > > OK. > > > Do you think we should rather be defensive and add some memsets, e.g. > > > > if (unlikely(i > SYSCALL_MAX_ARGS)) { > > WARN_ONCE(1, "i > SYSCALL_MAX_ARGS"); > > memset(args, 0, n * sizeof(args[0])); > > return; > > } > > if (unlikely(n > SYSCALL_MAX_ARGS - i)) { > > unsigned int extra = n - (SYSCALL_MAX_ARGS - i); > > > > WARN_ONCE(1, "i + n > SYSCALL_MAX_ARGS"); > > n = SYSCALL_MAX_ARGS - i; > > memset(&args[n], 0, extra * sizeof(args[0])); > > } > > ? > > Yes please. > > But please handle all of that in the generic code, so it doesn't have to be > replicated across all architectures. > > E.g. make syscall_get_arguments() a wrapper in generic code, calling > __syscall_get_arguments() in architecture-specific code. > > And make the latter return int, so it can indicate other failures. Other failures? What syscall_get_arguments is expected to do if __syscall_get_arguments returned, say, -1? Anyway, as asm-generic/syscall.h is used for documentation purposes only, I can try to prepare it for inclusion in other files without risk of starting a big refactoring that would affect all architectures -- a thing I'd rather not include into this series which is all about adding PTRACE_GET_SYSCALL_INFO API. I suppose there is no rush and arch-specific asm/syscall.h can switch to use asm-generic/syscall.h gradually. -- ldv