stapdyn: Batch _stp_strncpy_from_user reads

In order to reduce the number of syscalls required, this strncpy now
opportunistically reads larger blocks before checking for '\0'.  Reads
are kept within page boundaries to avoid running into invalid memory.

diff --git a/runtime/dyninst/copy.c b/runtime/dyninst/copy.c
index 54a3519137426ed107e0583155760c2e8c196657..4b284542e827601e0c8f1171a6c6764c94318a7c 100644 (file)
--- a/runtime/dyninst/copy.c
+++ b/runtime/dyninst/copy.c
@@ -50,16 +50,30 @@ static inline __must_check long __copy_to_user(void *to, const void *from,
 static long
 _stp_strncpy_from_user(char *dst, const char *src, long count)
 {
-       long i;
        if (count <= 0)
                return -EINVAL;
-       for (i = 0; i < count; ++i) {
-               if (__copy_from_user(dst, src++, 1))
+
+       /* Reads are batched on aligned 4k boundaries, approximately
+        * page size, to reduce the number of pread syscalls.  It will
+        * likely read past the terminating '\0', but shouldn't fault.
+        * NB: We shouldn't try to read the entire 'count' at once, in
+        * case some small string is already near the end of its page.
+        */
+       long i = 0;
+       while (i < count) {
+               long n = 0x1000 - ((long)(src + i) & 0xFFF);
+               n = min(n, count - i);
+               if (__copy_from_user(dst + i, src + i, n))
                        return -EFAULT;
-               if (*dst++ == 0)
-                       return i;
+
+               char *dst0 = memchr(dst + i, 0, n);
+               if (dst0 != NULL)
+                       return (dst0 - dst);
+
+               i += n;
        }
-       *(dst - 1) = 0;
+
+       dst[i - 1] = 0;
        return i - 1;
 }