Re: [PATCH] arm64/cpufeature: Move BUG_ON() inside get_arm64_ftr_reg()

[Will Deacon]

On Wed, May 20, 2020 at 04:47:11PM +0100, Catalin Marinas wrote:
> On Wed, May 20, 2020 at 01:20:13PM +0100, Will Deacon wrote:
> > On Wed, May 20, 2020 at 06:52:54AM +0530, Anshuman Khandual wrote:
> > > There is no way to proceed when requested register could not be searched in
> > > arm64_ftr_reg[]. Requesting for a non present register would be an error as
> > > well. Hence lets just BUG_ON() when the search fails in get_arm64_ftr_reg()
> > > rather than checking for return value and doing the same in some individual
> > > callers.
> > > 
> > > But there are some callers that dont BUG_ON() upon search failure. It adds
> > > an argument 'failsafe' that provides required switch between callers based
> > > on whether they could proceed or not.
> > > 
> > > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > > Cc: Will Deacon <will@xxxxxxxxxx>
> > > Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> > > Cc: Mark Brown <broonie@xxxxxxxxxx>
> > > Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > 
> > > Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> > > ---
> > > Applies on next-20200518 that has recent cpufeature changes from Will.
> > > 
> > >  arch/arm64/kernel/cpufeature.c | 26 +++++++++++++-------------
> > >  1 file changed, 13 insertions(+), 13 deletions(-)
> > > 
> > > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > > index bc5048f152c1..62767cc540c3 100644
> > > --- a/arch/arm64/kernel/cpufeature.c
> > > +++ b/arch/arm64/kernel/cpufeature.c
> > > @@ -557,7 +557,7 @@ static int search_cmp_ftr_reg(const void *id, const void *regp)
> > >   *         - NULL on failure. It is upto the caller to decide
> > >   *	     the impact of a failure.
> > >   */
> > > -static struct arm64_ftr_reg *get_arm64_ftr_reg(u32 sys_id)
> > > +static struct arm64_ftr_reg *get_arm64_ftr_reg(u32 sys_id, bool failsafe)
> > 
> > Generally, I'm not a big fan of boolean arguments because they are really
> > opaque at the callsite. It also seems bogus to me that we don't trust the
> > caller to pass a valid sys_id, but we trust it to get "failsafe" right,
> > which seems to mean "I promise to check the result isn't NULL before
> > dereferencing it."
> > 
> > So I don't see how this patch improves anything. I'd actually be more
> > inclined to stick a WARN() in get_arm64_ftr_reg() when it returns NULL and
> > have the callers handle NULL by returning early, getting rid of all the
> > BUG_ONs in here. Sure, the system might end up in a funny state, but we
> > WARN()d about it and tried to keep going (and Linus has some strong opinions
> > on this too).
> 
> Such WARN can be triggered by the user via emulate_sys_reg(), so we
> can't really have it in get_arm64_ftr_reg() without a 'failsafe' option.

Ah yes, that would be bad. In which case, I don't think the existing code
should change.

Will