Messages in this thread | | | Subject | Re: [PATCH v2 23/25] powerpc/signal: Create 'unsafe' versions of copy_[ck][fpr/vsx]_to_user() | From | Christophe Leroy <> | Date | Tue, 29 Sep 2020 07:33:30 +0200 |
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Le 29/09/2020 à 07:22, Christophe Leroy a écrit : > > > Le 29/09/2020 à 04:04, Christopher M. Riedl a écrit : >> On Tue Aug 18, 2020 at 12:19 PM CDT, Christophe Leroy wrote: >>> For the non VSX version, that's trivial. Just use unsafe_copy_to_user() >>> instead of __copy_to_user(). >>> >>> For the VSX version, remove the intermediate step through a buffer and >>> use unsafe_put_user() directly. This generates a far smaller code which >>> is acceptable to inline, see below: >>> >>> >>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu> >>> --- >>> arch/powerpc/kernel/signal.h | 53 ++++++++++++++++++++++++++++++++++++ >>> 1 file changed, 53 insertions(+) >>> >>> diff --git a/arch/powerpc/kernel/signal.h b/arch/powerpc/kernel/signal.h >>> index f610cfafa478..2559a681536e 100644 >>> --- a/arch/powerpc/kernel/signal.h >>> +++ b/arch/powerpc/kernel/signal.h >>> @@ -32,7 +32,54 @@ unsigned long copy_fpr_to_user(void __user *to, >>> struct task_struct *task); >>> unsigned long copy_ckfpr_to_user(void __user *to, struct task_struct >>> *task); >>> unsigned long copy_fpr_from_user(struct task_struct *task, void __user >>> *from); >>> unsigned long copy_ckfpr_from_user(struct task_struct *task, void __user >>> *from); >>> + >>> +#define unsafe_copy_fpr_to_user(to, task, label) do { \ >>> + struct task_struct *__t = task; \ >>> + u64 __user *buf = (u64 __user *)to; \ >>> + int i; \ >>> + \ >>> + for (i = 0; i < ELF_NFPREG - 1 ; i++) \ >>> + unsafe_put_user(__t->thread.TS_FPR(i), &buf[i], label); \ >>> + unsafe_put_user(__t->thread.fp_state.fpscr, &buf[i], label); \ >>> +} while (0) >>> + >> >> I've been working on the PPC64 side of this "unsafe" rework using this >> series as a basis. One question here - I don't really understand what >> the benefit of re-implementing this logic in macros (similarly for the >> other copy_* functions below) is? > > Not sure either. > > The whole purpose is to not manage the error through a local var but exclusively use labels. > However, GCC is probably smart enough to understand it and drop the local var while inlining. > > One important thing however is to make sure we won't end up with an outline function, otherwise you > completely loose the benefit of the label stuff. And you get a function call inside a user access, > which is what we want to avoid. > >> >> I am considering a "__unsafe_copy_*" implementation in signal.c for >> each (just the original implementation w/ using the "unsafe_" variants >> of the uaccess stuff) which gets called by the "safe" functions w/ the >> appropriate "user_*_access_begin/user_*_access_end". Something like >> (pseudo-ish code): > > Good idea, however ... > >> >> /* signal.c */ >> unsigned long __unsafe_copy_fpr_to_user(...) >> { >> ... >> unsafe_copy_to_user(..., bad); >> return 0; >> bad: >> return 1; /* -EFAULT? */ >> } > > This __unsafe_copy_fpr_to_user() has to be in signal.h and must be tagged 'static __always_inline' > for the reasons explained above. > >> >> unsigned long copy_fpr_to_user(...) >> { >> unsigned long err; >> if (!user_write_access_begin(...)) >> return 1; /* -EFAULT? */ >> >> err = __unsafe_copy_fpr_to_user(...); >> >> user_write_access_end(); >> return err; >> }
Also note that at the end (ie when both PPC32 and PPC64 signal code are using "unsafe" versions), the "safe" version won't be used anymore and will be dropped.
Christophe
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