From: Noah Goldstein <goldstein.w.n@gmail.com>
To: "H.J. Lu" <hjl.tools@gmail.com>
Cc: libc-alpha@sourceware.org, carlos@systemhalted.org
Subject: Re: [PATCH v1] x86: Fix strncat-avx2.S when `src` has no null-term [BZ #30065]
Date: Tue, 31 Jan 2023 16:52:20 -0600 [thread overview]
Message-ID: <CAFUsyf+W+hp=5J=ZotGhuFi-bxXkoOafrk0CO+AyapHuP7SRVw@mail.gmail.com> (raw)
In-Reply-To: <CAMe9rOqwg+uoBD7r_822UKcRAFJ-i=iBNW4aE5cmpfP7F_n6BA@mail.gmail.com>
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.
>
> > > > 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.
next prev parent reply other threads:[~2023-01-31 22:52 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-31 21:36 Noah Goldstein
2023-01-31 22:28 ` H.J. Lu
2023-01-31 22:37 ` Noah Goldstein
2023-01-31 22:42 ` H.J. Lu
2023-01-31 22:52 ` Noah Goldstein [this message]
2023-01-31 23:01 ` H.J. Lu
2023-01-31 23:09 ` Noah Goldstein
2023-01-31 23:40 ` H.J. Lu
2023-01-31 23:47 ` Noah Goldstein
2023-01-31 23:46 ` [PATCH v2] x86: Fix strncat-avx2.S reading past length " Noah Goldstein
2023-02-01 0:23 ` H.J. Lu
2023-02-01 3:10 ` Carlos O'Donell
2023-02-12 1:00 ` Sunil Pandey
2023-02-12 1:36 ` H.J. Lu
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