On Tue, Apr 13, 2021 at 7:20 PM Mathieu Desnoyers <mathieu.desnoy...@efficios.com> wrote: > > ----- On Apr 13, 2021, at 1:07 PM, Eric Dumazet eduma...@google.com wrote: > > > On Tue, Apr 13, 2021 at 7:01 PM Eric Dumazet <eduma...@google.com> wrote: > >> > >> On Tue, Apr 13, 2021 at 6:57 PM Eric Dumazet <eduma...@google.com> wrote: > >> > > >> > On Tue, Apr 13, 2021 at 6:54 PM Mathieu Desnoyers > >> > <mathieu.desnoy...@efficios.com> wrote: > >> > > > >> > > ----- On Apr 13, 2021, at 12:22 PM, Eric Dumazet > >> > > eric.duma...@gmail.com wrote: > >> > > > >> > > > From: Eric Dumazet <eduma...@google.com> > >> > > > > >> > > > Commit ec9c82e03a74 ("rseq: uapi: Declare rseq_cs field as union, > >> > > > update includes") added regressions for our servers. > >> > > > > >> > > > Using copy_from_user() and clear_user() for 64bit values > >> > > > is suboptimal. > >> > > > > >> > > > We can use faster put_user() and get_user(). > >> > > > > >> > > > 32bit arches can be changed to use the ptr32 field, > >> > > > since the padding field must always be zero. > >> > > > > >> > > > v2: added ideas from Peter and Mathieu about making this > >> > > > generic, since my initial patch was only dealing with > >> > > > 64bit arches. > >> > > > >> > > Ah, now I remember the reason why reading and clearing the entire > >> > > 64-bit > >> > > is important: it's because we don't want to allow user-space processes > >> > > to > >> > > use this change in behavior to figure out whether they are running on a > >> > > 32-bit or in a 32-bit compat mode on a 64-bit kernel. > >> > > > >> > > So although I'm fine with making 64-bit kernels faster, we'll want to > >> > > keep > >> > > updating the entire 64-bit ptr field on 32-bit kernels as well. > >> > > > >> > > Thanks, > >> > > > >> > > >> > So... back to V1 then ? > >> > >> Or add more stuff as in : > > > > diff against v2, WDYT ? > > I like this approach slightly better, because it moves the preprocessor > ifdefs into > rseq_get_rseq_cs and clear_rseq_cs, while keeping the same behavior for a > 32-bit > process running on native 32-bit kernel and as compat task on a 64-bit kernel. > > That being said, I don't expect anyone to care much about performance of > 32-bit > kernels, so we could use copy_from_user() on 32-bit kernels to remove > special-cases > in 32-bit specific code. This would eliminate the 32-bit specific "padding" > read, and > let the TASK_SIZE comparison handle the check for both 32-bit and 64-bit > kernels. > > As for clear_user(), I wonder whether we could simply keep using it, but > change the > clear_user() macro to figure out that it can use a faster 8-byte put_user ? I > find it > odd that performance optimizations which would be relevant elsewhere creep > into the > rseq code.
clear_user() is a maze of arch-dependent macros/functions/assembly I guess the same could be said from copy_in_user(), but apparently we removed special-casing, like in commit a41e0d754240fe8ca9c4f2070bf67e3b0228aa22 Definitely it seems odd having to carefully choose between multiple methods. > > Thanks, > > Mathieu > > > > > diff --git a/kernel/rseq.c b/kernel/rseq.c > > index > > f2eee3f7f5d330688c81cb2e57d47ca6b843873e..537b1f684efa11069990018ffa3642c209993011 > > 100644 > > --- a/kernel/rseq.c > > +++ b/kernel/rseq.c > > @@ -136,6 +136,10 @@ static int rseq_get_cs_ptr(struct rseq_cs __user > > **uptrp, > > { > > u32 ptr; > > > > + if (get_user(ptr, &rseq->rseq_cs.ptr.padding)) > > + return -EFAULT; > > + if (ptr) > > + return -EINVAL; > > if (get_user(ptr, &rseq->rseq_cs.ptr.ptr32)) > > return -EFAULT; > > *uptrp = (struct rseq_cs __user *)ptr; > > @@ -150,8 +154,9 @@ static int rseq_get_rseq_cs(struct task_struct *t, > > struct rseq_cs *rseq_cs) > > u32 sig; > > int ret; > > > > - if (rseq_get_cs_ptr(&urseq_cs, t->rseq)) > > - return -EFAULT; > > + ret = rseq_get_cs_ptr(&urseq_cs, t->rseq); > > + if (ret) > > + return ret; > > if (!urseq_cs) { > > memset(rseq_cs, 0, sizeof(*rseq_cs)); > > return 0; > > @@ -237,7 +242,8 @@ static int clear_rseq_cs(struct task_struct *t) > > #ifdef CONFIG_64BIT > > return put_user(0UL, &t->rseq->rseq_cs.ptr64); > > #else > > - return put_user(0UL, &t->rseq->rseq_cs.ptr.ptr32); > > + return put_user(0UL, &t->rseq->rseq_cs.ptr.ptr32) | > > + put_user(0UL, &t->rseq->rseq_cs.ptr.padding); > > #endif > > } > > -- > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com