x86: Fix page cross case in rawmemchr-avx2 [BZ #29234]

commit 6dcbb7d95d
Author: Noah Goldstein <goldstein.w.n@gmail.com>
Date:   Mon Jun 6 21:11:33 2022 -0700

    x86: Shrink code size of memchr-avx2.S

Changed how the page cross case aligned string (rdi) in
rawmemchr. This was incompatible with how
`L(cross_page_continue)` expected the pointer to be aligned and
would cause rawmemchr to read data start started before the
beginning of the string. What it would read was in valid memory
but could count CHAR matches resulting in an incorrect return
value.

This commit fixes that issue by essentially reverting the changes to
the L(page_cross) case as they didn't really matter.

Test cases added and all pass with the new code (and where confirmed
to fail with the old code).
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

string/test-rawmemchr.c

@@ -17,6 +17,7 @@
    <https://www.gnu.org/licenses/>.  */
 
 #include <assert.h>
+#include <support/xunistd.h>
 
 #define TEST_MAIN
 #define TEST_NAME "rawmemchr"
@@ -50,13 +51,45 @@ do_one_test (impl_t *impl, const char *s, int c, char *exp_res)
     }
 }
 
+static void
+do_test_bz29234 (void)
+{
+  size_t i, j;
+  char *ptr_start;
+  char *buf = xmmap (0, 8192, PROT_READ | PROT_WRITE,
+		     MAP_PRIVATE | MAP_ANONYMOUS, -1);
+
+  memset (buf, -1, 8192);
+
+  ptr_start = buf + 4096 - 8;
+
+  /* Out of range matches before the start of a page. */
+  memset (ptr_start - 8, 0x1, 8);
+
+  for (j = 0; j < 8; ++j)
+    {
+      for (i = 0; i < 128; ++i)
+	{
+	  ptr_start[i + j] = 0x1;
+
+	  FOR_EACH_IMPL (impl, 0)
+	    do_one_test (impl, (char *) (ptr_start + j), 0x1,
+			 ptr_start + i + j);
+
+	  ptr_start[i + j] = 0xff;
+	}
+    }
+
+  xmunmap (buf, 8192);
+}
+
 static void
 do_test (size_t align, size_t pos, size_t len, int seek_char)
 {
   size_t i;
   char *result;
 
-  align &= 7;
+  align &= getpagesize () - 1;
   if (align + len >= page_size)
     return;
 
@@ -114,6 +147,13 @@ do_random_tests (void)
 	    }
 	}
 
+      if (align)
+	{
+	  p[align - 1] = seek_char;
+	  if (align > 4)
+	    p[align - 4] = seek_char;
+	}
+
       assert (pos < len);
       size_t r = random ();
       if ((r & 31) == 0)
@@ -129,6 +169,13 @@ do_random_tests (void)
 		   result, p);
 	    ret = 1;
 	  }
+
+      if (align)
+	{
+	  p[align - 1] = seek_char;
+	  if (align > 4)
+	    p[align - 4] = seek_char;
+	}
     }
 }
 
@@ -150,14 +197,22 @@ test_main (void)
       do_test (i, 64, 256, 23);
       do_test (0, 16 << i, 2048, 0);
       do_test (i, 64, 256, 0);
+
+      do_test (getpagesize () - i, 64, 256, 23);
+      do_test (getpagesize () - i, 64, 256, 0);
     }
   for (i = 1; i < 32; ++i)
     {
       do_test (0, i, i + 1, 23);
       do_test (0, i, i + 1, 0);
+
+      do_test (getpagesize () - 7, i, i + 1, 23);
+      do_test (getpagesize () - i / 2, i, i + 1, 23);
+      do_test (getpagesize () - i, i, i + 1, 23);
     }
 
   do_random_tests ();
+  do_test_bz29234 ();
   return ret;
 }