[v2] checkpatch: Don't emit spurious warnings about block comments

In checkpatch we attempt to check for and warn about
block comments which start with /* or /** followed by a
non-blank. Unfortunately a bug in the regex meant that
we would incorrectly warn about comments starting with
"/**" with no following text:

  git show 9813dc6ac3954d58ba16b3920556f106f97e1c67|./scripts/checkpatch.pl -
  WARNING: Block comments use a leading /* on a separate line
  #34: FILE: tests/libqtest.h:233:
  +/**

The sequence "/\*\*?" was intended to match either "/*" or "/**",
but Perl's semantics for '?' allow it to backtrack and try the
"matches 0 chars" option if the "matches 1 char" choice leads to
a failure of the rest of the regex to match.  Switch to "/\*\*?+"
which uses what perlre(1) calls the "possessive" quantifier form:
this means that if it matches the "/**" string it will not later
backtrack to matching just the "/*" prefix.

The other end of the regex is also wrong: it is attempting
to check for "/* or /** followed by something that isn't
just whitespace", but [ \t]*.+[ \t]* will match on pure
whitespace. This is less significant but means that a line
with just a comment-starter followed by trailing whitespace
will generate an incorrect warning about block comment style
as well as the correct error about trailing whitespace which
a different checkpatch test emits.

Reported-by: Thomas Huth <thuth@redhat.com>
Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
This comment check is unique to QEMU checkpatch so the bugs
don't exist in the Linux version.

v1->v2 changes: Add the fix to the other end of the regex
pointed out by Eric, so that we don't emit spurious warnings
for block comment starters with trailing whitespace.

---
 scripts/checkpatch.pl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

On 1/18/19 10:50 AM, Peter Maydell wrote:
> In checkpatch we attempt to check for and warn about
> block comments which start with /* or /** followed by a
> non-blank. Unfortunately a bug in the regex meant that
> we would incorrectly warn about comments starting with
> "/**" with no following text:
> 
>   git show 9813dc6ac3954d58ba16b3920556f106f97e1c67|./scripts/checkpatch.pl -
>   WARNING: Block comments use a leading /* on a separate line
>   #34: FILE: tests/libqtest.h:233:
>   +/**
> 
> The sequence "/\*\*?" was intended to match either "/*" or "/**",
> but Perl's semantics for '?' allow it to backtrack and try the
> "matches 0 chars" option if the "matches 1 char" choice leads to
> a failure of the rest of the regex to match.  Switch to "/\*\*?+"
> which uses what perlre(1) calls the "possessive" quantifier form:
> this means that if it matches the "/**" string it will not later
> backtrack to matching just the "/*" prefix.
> 
> The other end of the regex is also wrong: it is attempting
> to check for "/* or /** followed by something that isn't
> just whitespace", but [ \t]*.+[ \t]* will match on pure
> whitespace. This is less significant but means that a line
> with just a comment-starter followed by trailing whitespace
> will generate an incorrect warning about block comment style
> as well as the correct error about trailing whitespace which
> a different checkpatch test emits.
> 

Fixes: 8c06fbdf36bf4d4d486116200248730887a4d7d6

> Reported-by: Thomas Huth <thuth@redhat.com>
> Reported-by: Eric Blake <eblake@redhat.com>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---

>  		# Block comments use /* on a line of its own
>  		if ($rawline !~ m@^\+.*/\*.*\*/[ \t]*$@ &&	#inline /*...*/
> -		    $rawline =~ m@^\+.*/\*\*?[ \t]*.+[ \t]*$@) { # /* or /** non-blank
> +		    $rawline =~ m@^\+.*/\*\*?+[ \t]*[^ \t]@) { # /* or /** non-blank

Reviewed-by: Eric Blake <eblake@redhat.com>

On Fri, 18 Jan 2019 at 17:06, Eric Blake <eblake@redhat.com> wrote:
>
> On 1/18/19 10:50 AM, Peter Maydell wrote:
> > In checkpatch we attempt to check for and warn about
> > block comments which start with /* or /** followed by a
> > non-blank. Unfortunately a bug in the regex meant that
> > we would incorrectly warn about comments starting with
> > "/**" with no following text:
> >
> >   git show 9813dc6ac3954d58ba16b3920556f106f97e1c67|./scripts/checkpatch.pl -
> >   WARNING: Block comments use a leading /* on a separate line
> >   #34: FILE: tests/libqtest.h:233:
> >   +/**
> >
> > The sequence "/\*\*?" was intended to match either "/*" or "/**",
> > but Perl's semantics for '?' allow it to backtrack and try the
> > "matches 0 chars" option if the "matches 1 char" choice leads to
> > a failure of the rest of the regex to match.  Switch to "/\*\*?+"
> > which uses what perlre(1) calls the "possessive" quantifier form:
> > this means that if it matches the "/**" string it will not later
> > backtrack to matching just the "/*" prefix.
> >
> > The other end of the regex is also wrong: it is attempting
> > to check for "/* or /** followed by something that isn't
> > just whitespace", but [ \t]*.+[ \t]* will match on pure
> > whitespace. This is less significant but means that a line
> > with just a comment-starter followed by trailing whitespace
> > will generate an incorrect warning about block comment style
> > as well as the correct error about trailing whitespace which
> > a different checkpatch test emits.
> >
>
> Fixes: 8c06fbdf36bf4d4d486116200248730887a4d7d6
>
> > Reported-by: Thomas Huth <thuth@redhat.com>
> > Reported-by: Eric Blake <eblake@redhat.com>
> > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> > ---
>
> >               # Block comments use /* on a line of its own
> >               if ($rawline !~ m@^\+.*/\*.*\*/[ \t]*$@ &&      #inline /*...*/
> > -                 $rawline =~ m@^\+.*/\*\*?[ \t]*.+[ \t]*$@) { # /* or /** non-blank
> > +                 $rawline =~ m@^\+.*/\*\*?+[ \t]*[^ \t]@) { # /* or /** non-blank
>
> Reviewed-by: Eric Blake <eblake@redhat.com>

Thanks. I'll take this via target-arm.next, just for convenience...

-- PMM