Re: [PATCH 1/2] riscv: allow case-insensitive ISA string parsing

[Andrew Jones]

On Thu, Apr 27, 2023 at 10:04:34AM +0100, Conor Dooley wrote:
> On Thu, Apr 27, 2023 at 09:53:19AM +0200, Andrew Jones wrote:
> > On Wed, Apr 26, 2023 at 07:54:39PM +0100, Conor Dooley wrote:
> > > (+CC Drew)
> > > 
> > > Hey Yangyu,
> > > 
> > > One meta-level comment - can you submit this patch + my dt-bindings
> > > patch as a v2?
> > > Some comments below.
> > > 
> > > On Tue, Apr 25, 2023 at 08:00:15PM +0800, Yangyu Chen wrote:
> > > > According to RISC-V ISA specification, the ISA naming strings are case
> > > > insensitive. The kernel docs require the riscv,isa string must be all
> > > > lowercase to simplify parsing currently. However, this limitation is not
> > > > consistent with RISC-V ISA Spec.
> > > 
> > > Please remove the above and cite ACPI's case-insensitivity as the
> > > rationale for this change.
> > > 
> > > > This patch modifies the ISA string parser in the kernel to support
> > > > case-insensitive ISA string parsing. It replaces `strncmp` with
> > > > `strncasecmp`, replaces `islower` with `isalpha`, and wraps the
> > > > dereferenced char in the parser with `tolower`.
> > > > 
> > > > Signed-off-by: Yangyu Chen <cyy@xxxxxxxxxxxx>
> > > > ---
> > > >  arch/riscv/kernel/cpu.c        |  6 ++++--
> > > >  arch/riscv/kernel/cpufeature.c | 20 ++++++++++----------
> > > >  2 files changed, 14 insertions(+), 12 deletions(-)
> > > > 
> > > > diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
> > > > index 8400f0cc9704..531c76079b73 100644
> > > > --- a/arch/riscv/kernel/cpu.c
> > > > +++ b/arch/riscv/kernel/cpu.c
> > > > @@ -4,6 +4,7 @@
> > > >   */
> > > >  
> > > >  #include <linux/cpu.h>
> > > > +#include <linux/ctype.h>
> > > >  #include <linux/init.h>
> > > >  #include <linux/seq_file.h>
> > > >  #include <linux/of.h>
> > > > @@ -41,7 +42,7 @@ int riscv_of_processor_hartid(struct device_node *node, unsigned long *hart)
> > > >  		pr_warn("CPU with hartid=%lu has no \"riscv,isa\" property\n", *hart);
> > > >  		return -ENODEV;
> > > >  	}
> > > > -	if (isa[0] != 'r' || isa[1] != 'v') {
> > > > +	if (tolower(isa[0]) != 'r' || tolower(isa[1]) != 'v') {
> > > >  		pr_warn("CPU with hartid=%lu has an invalid ISA of \"%s\"\n", *hart, isa);
> > > >  		return -ENODEV;
> > > 
> > > I don't understand why this is even here in the first place. I'd be
> > > inclined to advocate for it's entire removal. Checking *only* that there
> > > is an "rv" in that string seems pointless to me. If you're on a 64-bit
> > > kernel and the node has riscv,isa = "rv32ima" it's gonna say it is okay?
> > > Drew what do you think?
> > 
> > It makes some sense to me as a garbage detector. It's unlikely the first
> > two bytes will be "rv" if the string is random junk.
> 
> Preventing the input of absolute rubbish is dt-validate's job & if the dtb
> itself has been corrupted somehow I suspect that we have bigger problems
> than checking for "rv" will solve.

We would, but would they be as easy to debug as this very early sanity
check? I agree, though, that doing the sanity checking in
riscv_of_processor_hartid(), which gets called from several different
places, seems a bit much. It'd be better to do that once, early, and
never again.

> 
> > also do a strlen(isa) >= 4 check first, though. of_property_read_string()
> > will succeed even when the string is "".
> 
> I don't think that checking that there are at least 4 characters isn't
> even sufficient. Either we should confirm that this is a valid riscv,isa
> to run on (so rv##ima w/ ## matching the kernel) or not bother at all.

Extending the check makes sense, but even more reason to do it outside
riscv_of_processor_hartid().

> 
> It's a different issue though, and I'd be inclined to revisit it in the
> future when the ACPI stuff is in, along with perhaps the cleanup parts
> of Heiko's series too.

Agreed.

Thanks,
drew