Re: [PATCH] x86/cpuid: Calculate FEATURESET_NR_ENTRIES more helpfully

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.05.2023 14:45, Andrew Cooper wrote:
> When adding new featureset words, it is convenient to split the work into
> several patches.  However, GCC 12 spotted that the way we prefer to split the
> work results in a real (transient) breakage whereby the policy <-> featureset
> helpers perform out-of-bounds accesses on the featureset array.
> 
> Fix this by having gen-cpuid.py calculate FEATURESET_NR_ENTRIES from the
> comments describing the word blocks, rather than from the XEN_CPUFEATURE()
> with the greatest value.
> 
> For simplicty, require that the word blocks appear in order.  This can be
> revisted if we find a good reason to have blocks out of order.
> 
> No functional change.
> 
> Reported-by: Jan Beulich <jbeulich@xxxxxxxx>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

As far as my Python goes:
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Just one remark further down.

> This supercedes the entire "x86: Fix transient build breakage with featureset
> additions" series, but doesn't really feel as if it ought to be labelled v2

Thank you for re-doing this altogether. I think it's more safe this way,
and it now being less intrusive is imo also beneficial.

> --- a/xen/tools/gen-cpuid.py
> +++ b/xen/tools/gen-cpuid.py
> @@ -50,13 +50,37 @@ def parse_definitions(state):
>          "\s+([\s\d]+\*[\s\d]+\+[\s\d]+)\)"
>          "\s+/\*([\w!]*) .*$")
>  
> +    word_regex = re.compile(
> +        r"^/\* .* word (\d*) \*/$")
> +    last_word = -1
> +
>      this = sys.modules[__name__]
>  
>      for l in state.input.readlines():
> -        # Short circuit the regex...
> -        if not l.startswith("XEN_CPUFEATURE("):
> +
> +        # Short circuit the regexes...
> +        if not (l.startswith("XEN_CPUFEATURE(") or
> +                l.startswith("/* ")):
>              continue
>  
> +        # Handle /* ... word $N */ lines
> +        if l.startswith("/* "):
> +
> +            res = word_regex.match(l)
> +            if res is None:
> +                continue # Some other comment
> +
> +            word = int(res.groups()[0])
> +
> +            if word != last_word + 1:
> +                raise Fail("Featureset word %u out of order (last word %u)"
> +                           % (word, last_word))
> +
> +            last_word = word
> +            state.nr_entries = word + 1
> +            continue
> +
> +        # Handle XEN_CPUFEATURE( lines
>          res = feat_regex.match(l)
>  
>          if res is None:
> @@ -94,6 +118,15 @@ def parse_definitions(state):
>      if len(state.names) == 0:
>          raise Fail("No features found")
>  
> +    if state.nr_entries == 0:
> +        raise Fail("No featureset word info found")
> +
> +    max_val = max(state.names.keys())
> +    if (max_val >> 5) + 1 > state.nr_entries:

Maybe

    if (max_val >> 5) >= state.nr_entries:

?

Jan