[PATCH 3/9] Avoid undefined behavior in extract_integer

[Tom Tromey <tom at tromey dot com>]

-fsanitize=undefined showed that extract_integer could left-shift a
negative value, which is undefined.  This patch fixes the problem by
doing all the work in an unsigned type, and then using a static_cast
at the end of the function.  This relies on implementation-defined
behavior, but I tend to think we are on safe ground there.  (Also, if
need be, violations of this could probably be detected, either by
configure or by a static_assert.)

ChangeLog
2018-08-27  Tom Tromey  <tom@tromey.com>

	* findvar.c (extract_integer): Do work in an unsigned type and
	cast at the end.
---
 gdb/ChangeLog | 5 +++++
 gdb/findvar.c | 4 ++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/gdb/findvar.c b/gdb/findvar.c
index 9256833ab60..f2b84db82a1 100644
--- a/gdb/findvar.c
+++ b/gdb/findvar.c
@@ -50,7 +50,7 @@ template<typename T, typename>
 T
 extract_integer (const gdb_byte *addr, int len, enum bfd_endian byte_order)
 {
-  T retval = 0;
+  typename std::make_unsigned<T>::type retval = 0;
   const unsigned char *p;
   const unsigned char *startaddr = addr;
   const unsigned char *endaddr = startaddr + len;
@@ -86,7 +86,7 @@ That operation is not available on integers of more than %d bytes."),
       for (; p >= startaddr; --p)
 	retval = (retval << 8) | *p;
     }
-  return retval;
+  return static_cast<T> (retval);
 }
 
 /* Explicit instantiations.  */
-- 
2.13.6