[PATCH v2 07/10] Avoid undefined behavior in read_signed_leb128

[Tom Tromey <tom@tromey.com>]

-fsanitize=undefined pointed out that read_signed_leb128 had an
undefined left-shift when processing the final byte of a 64-bit leb:

    runtime error: left shift of 127 by 63 places cannot be represented in type 'long int'

and an undefined negation:

    runtime error: negation of -9223372036854775808 cannot be represented in type 'long int'; cast to an unsigned type to negate this value to itself

Both of these problems are readily avoided by havinng
read_signed_leb128 work in an unsigned type, and then casting to the
signed type at the return.

gdb/ChangeLog
2018-10-01  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (read_signed_leb128): Work in ULONGEST.
---
 gdb/ChangeLog    | 4 ++++
 gdb/dwarf2read.c | 6 +++---
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 4013c199da..7004299a91 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -19627,7 +19627,7 @@ static LONGEST
 read_signed_leb128 (bfd *abfd, const gdb_byte *buf,
 		    unsigned int *bytes_read_ptr)
 {
-  LONGEST result;
+  ULONGEST result;
   int shift, num_read;
   unsigned char byte;
 
@@ -19639,7 +19639,7 @@ read_signed_leb128 (bfd *abfd, const gdb_byte *buf,
       byte = bfd_get_8 (abfd, buf);
       buf++;
       num_read++;
-      result |= ((LONGEST) (byte & 127) << shift);
+      result |= ((ULONGEST) (byte & 127) << shift);
       shift += 7;
       if ((byte & 128) == 0)
 	{
@@ -19647,7 +19647,7 @@ read_signed_leb128 (bfd *abfd, const gdb_byte *buf,
 	}
     }
   if ((shift < 8 * sizeof (result)) && (byte & 0x40))
-    result |= -(((LONGEST) 1) << shift);
+    result |= -(((ULONGEST) 1) << shift);
   *bytes_read_ptr = num_read;
   return result;
 }
-- 
2.17.1