Re: [PATCH v1] x86: Fix strncat-avx2.S when `src` has no null-term [BZ #30065]

["H.J. Lu" <hjl.tools@gmail.com>]

On Tue, Jan 31, 2023 at 2:52 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Tue, Jan 31, 2023 at 4:43 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > On Tue, Jan 31, 2023 at 2:37 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> > >
> > > On Tue, Jan 31, 2023 at 4:29 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> > > >
> > > > On Tue, Jan 31, 2023 at 1:37 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> > > > >
> > > > > Two issue:
> > > > >
> > > > > 1) Zero-length check is doing:
> > > > > ```
> > > > >     test    %rdx, %rdx
> > > > >     jl      L(zero_len)
> > > > > ```
> > > > > which doesn't actually check zero (was at some point `decq` and the
> > > > > flag never got updated).
> > > > >
> > > > > The fix is just make the flag `jle` i.e:
> > > > > ```
> > > > >     test    %rdx, %rdx
> > > > >     jle     L(zero_len)
> > > > > ```
> > > > >
> > > > > 2) Length check in page-cross case checking if we should continue is
> > > > > doing:
> > > > > ```
> > > > >     cmpq    %r8, %rdx
> > > > >     jb      L(page_cross_small)
> > > > > ```
> > > > > which means we will continue searching for null-term if length ends at
> > > > > the end of a page and there was no null-term in `src`.
> > > >
> > > > It is not purely about null-term.   strncat shouldn't read beyond the limit.
> > > > In this case, src may point to PROT_NONE memory.
> > > >
> > >
> > > If there was a null-term the later check:
> > > ```
> > > shll $CHAR_SIZE, %ecx
> > > jz L(page_cross_continue)
> > > ```
> > > would catch it, this can only occur if there is no null-term.
> >
> > But it is incorrect to read beyond the limit even if there is a null-term.
> It's a page-aligned read where len != 0 so the original read should be fine.
>
> The `jb L(page_cross_small)`
> falls through to the null-term check, so if there was a null-term we won't
> read anymore bytes.

There is no need to search null-term for zero length case.

> >
> > > > > The fix is to make the flag:
> > > > > ```
> > > > >     cmpq    %r8, %rdx
> > > > >     jbe     L(page_cross_small)
> > > > > ```
> > > > > ---
> > > > >  string/test-strncat.c                   | 25 ++++++++++++++++++++++++-
> > > > >  sysdeps/x86_64/multiarch/strncat-avx2.S |  4 ++--
> > > > >  2 files changed, 26 insertions(+), 3 deletions(-)
> > > > >
> > > > > diff --git a/string/test-strncat.c b/string/test-strncat.c
> > > > > index e03d329e1c..c0cde206ee 100644
> > > > > --- a/string/test-strncat.c
> > > > > +++ b/string/test-strncat.c
> > > > > @@ -28,6 +28,7 @@
> > > > >  # define CHAR char
> > > > >  # define UCHAR unsigned char
> > > > >  # define SIMPLE_STRNCAT simple_strncat
> > > > > +# define STRNLEN strnlen
> > > > >  # define STRLEN strlen
> > > > >  # define MEMSET memset
> > > > >  # define MEMCPY memcpy
> > > > > @@ -40,6 +41,7 @@
> > > > >  # define CHAR wchar_t
> > > > >  # define UCHAR wchar_t
> > > > >  # define SIMPLE_STRNCAT simple_wcsncat
> > > > > +# define STRNLEN wcsnlen
> > > > >  # define STRLEN wcslen
> > > > >  # define MEMSET wmemset
> > > > >  # define MEMCPY wmemcpy
> > > > > @@ -78,7 +80,7 @@ do_one_test (impl_t *impl, CHAR *dst, const CHAR *src, size_t n)
> > > > >        return;
> > > > >      }
> > > > >
> > > > > -  size_t len = STRLEN (src);
> > > > > +  size_t len = STRNLEN (src, n);
> > > > >    if (MEMCMP (dst + k, src, len + 1 > n ? n : len + 1) != 0)
> > > >
> > > > >      {
> > > > >        error (0, 0, "Incorrect concatenation in function %s",
> > > > > @@ -95,6 +97,26 @@ do_one_test (impl_t *impl, CHAR *dst, const CHAR *src, size_t n)
> > > > >      }
> > > > >  }
> > > > >
> > > > > +static void
> > > > > +do_test_src_no_nullterm_bz30065 (void)
> > > > > +{
> > > > > +  /* NB: "src does not need to be null-terminated if it contains n or more
> > > > > +   * bytes." */
> > > > > +  CHAR *s1, *s2;
> > > > > +  size_t bound = page_size / sizeof (CHAR);
> > > > > +  s1 = (CHAR *) (buf1 + BUF1PAGES * page_size);
> > > > > +  s2 = (CHAR *) buf2;
> > > > > +  MEMSET (s1 - bound, -1, bound);
> > > > > +  for (size_t n = 0; n < bound; ++n)
> > > > > +    {
> > > > > +      FOR_EACH_IMPL (impl, 0)
> > > > > +       {
> > > > > +         s2[0] = '\0';
> > > > > +         do_one_test (impl, s2, s1 - n, n);
> > > > > +       }
> > > > > +    }
> > > > > +}
> > > > > +
> > > > >  static void
> > > > >  do_test (size_t align1, size_t align2, size_t len1, size_t len2,
> > > > >          size_t n, int max_char)
> > > > > @@ -372,6 +394,7 @@ test_main (void)
> > > > >
> > > > >    do_random_tests ();
> > > > >    do_overflow_tests ();
> > > > > +  do_test_src_no_nullterm_bz30065 ();
> > > > >    return ret;
> > > > >  }
> > > > >
> > > > > diff --git a/sysdeps/x86_64/multiarch/strncat-avx2.S b/sysdeps/x86_64/multiarch/strncat-avx2.S
> > > > > index b380e8e11c..c2ff202238 100644
> > > > > --- a/sysdeps/x86_64/multiarch/strncat-avx2.S
> > > > > +++ b/sysdeps/x86_64/multiarch/strncat-avx2.S
> > > > > @@ -66,7 +66,7 @@ ENTRY(STRNCAT)
> > > > >         salq    $2, %rdx
> > > > >  # else
> > > > >         test    %rdx, %rdx
> > > > > -       jl      L(zero_len)
> > > > > +       jle     L(zero_len)
> > > > >  # endif
> > > > >         vpxor   %VZERO_128, %VZERO_128, %VZERO_128
> > > > >
> > > > > @@ -387,7 +387,7 @@ L(page_cross):
> > > > >         subl    %esi, %r8d
> > > > >         andl    $(VEC_SIZE - 1), %r8d
> > > > >         cmpq    %r8, %rdx
> > > > > -       jb      L(page_cross_small)
> > > > > +       jbe     L(page_cross_small)
> > > > >
> > > > >         /* Optimizing more aggressively for space as this is very cold
> > > > >            code. This saves 2x cache lines.  */
> > > > > --
> > > > > 2.34.1
> > > > >
> > > >
> > > >
> > > > --
> > > > H.J.
> >
> >
> >
> > --
> > H.J.



-- 
H.J.