* Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote: > rseq_abi()->flags and rseq_abi()->rseq_cs->flags 29 upper bits are > currently unused. > > The current behavior when those bits are set is to ignore them. This is > not an ideal behavior, because when future features will start using > those flags, if user-space fails to correctly validate that the kernel > indeed supports those flags (e.g. with a new sys_rseq flags bit) before > using them, it may incorrectly assume that the kernel will handle those > flags way when in fact those will be silently ignored on older kernels. > > Validating that unused flags bits are cleared will allow a smoother > transition when those flags will start to be used by allowing > applications to fail early, and obviously, when they attempt to use the > new flags on an older kernel that does not support them. > > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> > --- > kernel/rseq.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/kernel/rseq.c b/kernel/rseq.c > index 81d7dc80787b..bda8175f8f99 100644 > --- a/kernel/rseq.c > +++ b/kernel/rseq.c > @@ -176,7 +176,7 @@ static int rseq_need_restart(struct task_struct *t, u32 cs_flags) > u32 flags, event_mask; > int ret; > > - if (WARN_ON_ONCE(cs_flags & RSEQ_CS_NO_RESTART_FLAGS)) > + if (WARN_ON_ONCE(cs_flags & RSEQ_CS_NO_RESTART_FLAGS) || cs_flags) > return -EINVAL; > > /* Get thread flags. */ > @@ -184,7 +184,7 @@ static int rseq_need_restart(struct task_struct *t, u32 cs_flags) > if (ret) > return ret; > > - if (WARN_ON_ONCE(flags & RSEQ_CS_NO_RESTART_FLAGS)) > + if (WARN_ON_ONCE(flags & RSEQ_CS_NO_RESTART_FLAGS) || flags) > return -EINVAL; Just to make it clear: no existing libraries/tooling out there have learned to rely on the old ABI that ignored unset flags, right? Only then is this patch ABI-safe. Thanks, Ingo
