On Wed, 25 Mar 2020, Jakub Jelinek wrote:

> Hi!
> 
> The following testcase is miscompiled, because there is a buffer overflow
> in push_partial_def in the little-endian case when working 64-byte vectors.
> The code computes the number of bytes we need in the BUFFER: NEEDED_LEN,
> which is rounded up number of bits we need.  Then the code
> native_encode_expr each (partially overlapping) pd into THIS_BUFFER.
> If pd.offset < 0, i.e. the pd.rhs store starts at some bits before the
> window we are interested in, we pass -pd.offset to native_encode_expr and
> shrink the size already earlier:
>       HOST_WIDE_INT size = pd.size;
>       if (pd.offset < 0)
>         size -= ROUND_DOWN (-pd.offset, BITS_PER_UNIT);
> On this testcase, the problem is with a store with pd.offset > 0,
> in particular pd.offset 256, pd.size 512, i.e. a 64-byte store which doesn't
> fit into entirely into BUFFER.
> We have just:
>           size = MIN (size, (HOST_WIDE_INT) needed_len * BITS_PER_UNIT);
> in this case for little-endian, which isn't sufficient, because needed_len
> is 64, the entire BUFFER (except of the last extra byte used for shifting).
> native_encode_expr fills the whole THIS_BUFFER (again, except the last extra
> byte), and the code then performs memcpy (BUFFER + 32, THIS_BUFFER, 64);
> which overflows BUFFER and as THIS_BUFFER is usually laid out after it,
> overflows it into THIS_BUFFER.
> The following patch fixes it by for pd.offset > 0 making sure size is
> reduced too.  For big-endian the code does things differently and already
> handles this right.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

OK.

Thanks,
Richard.

> 2020-03-25  Jakub Jelinek  <ja...@redhat.com>
> 
>       PR tree-optimization/94300
>       * tree-ssa-sccvn.c (vn_walk_cb_data::push_partial_def): If pd.offset
>       is positive, make sure that off + size isn't larger than needed_len.
> 
>       * gcc.target/i386/avx512f-pr94300.c: New test.
> 
> --- gcc/tree-ssa-sccvn.c.jj   2020-03-12 14:28:13.365363769 +0100
> +++ gcc/tree-ssa-sccvn.c      2020-03-24 15:48:22.465737253 +0100
> @@ -2058,6 +2058,8 @@ vn_walk_cb_data::push_partial_def (pd_da
>               shift_bytes_in_array_left (this_buffer, len + 1, amnt);
>             unsigned int off = pd.offset / BITS_PER_UNIT;
>             gcc_assert (off < needed_len);
> +           size = MIN (size,
> +                       (HOST_WIDE_INT) (needed_len - off) * BITS_PER_UNIT);
>             p = buffer + off;
>             if (amnt + size < BITS_PER_UNIT)
>               {
> --- gcc/testsuite/gcc.target/i386/avx512f-pr94300.c.jj        2020-03-24 
> 15:38:32.391643991 +0100
> +++ gcc/testsuite/gcc.target/i386/avx512f-pr94300.c   2020-03-24 
> 15:39:09.874078217 +0100
> @@ -0,0 +1,21 @@
> +/* PR tree-optimization/94300 */
> +/* { dg-do run { target { avx512f } } } */
> +/* { dg-options "-O1 -mavx512f -mprefer-vector-width=512 
> -mtune=skylake-avx512" } */
> +
> +#include "avx512f-check.h"
> +
> +typedef double V __attribute__((vector_size (64)));
> +
> +static void
> +avx512f_test (void)
> +{
> +  double mem[16];
> +  const V a = { 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0 };
> +  const V b = { 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0 };
> +  V c;
> +  __builtin_memcpy (mem, &a, 64);
> +  __builtin_memcpy (mem + 8, &b, 64);
> +  __builtin_memcpy (&c, mem + 4, 64);
> +  if (c[5] != 9.0)
> +    __builtin_abort ();
> +}
> 
>       Jakub
> 
> 

-- 
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imend├Ârffer; HRB 36809 (AG Nuernberg)

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