[binutils-gdb] gdb: fix undefined behavior reported in copy_bitwise

[binutils-gdb] gdb: fix undefined behavior reported in copy_bitwise

[Simon Marchi]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=cf83625da29d1239e97f1eb4d145f347cb741889

commit cf83625da29d1239e97f1eb4d145f347cb741889
Author: Artur Shepilko <nomadbyte@gmail.com>
Date:   Fri Apr 10 10:56:43 2020 -0400

    gdb: fix undefined behavior reported in copy_bitwise
    
    gdb version 9.1, built with clang 8.0.0 on Ubuntu 18.04 (x86_64);
    --enable-ubsan (for clang's undefined behavior sanitizer)
    
    Executing command; `maint selftest copy_bitwise` bombs in runtime error:
    ../../gdb/utils.c:3432:28: runtime error: left shift of negative value -1
    
    Closer look reveals the offending shift: `(~0 << nbits)`, apparently 0
    is treated as signed int, resulting in negative complement. Explicitly
    stating it unsigned 0U  fixes it and the `copy_bitwise` test passes
    ok.

Diff:
---
 gdb/ChangeLog | 5 +++++
 gdb/utils.c   | 2 +-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 661a41467bb..81102ee569b 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@
+2020-04-10  Artur Shepilko  <nomadbyte@gmail.com>
+
+	* utils.c (copy_bitwise): Use unsigned 0 constant as operand of
+	bit shift.
+
 2020-04-10  Tom Tromey  <tromey@adacore.com>
 
 	* symfile.c (symbol_file_add_separate): Preserve OBJF_MAINLINE.
diff --git a/gdb/utils.c b/gdb/utils.c
index bda6bbf5b0e..f5b20331b1e 100644
--- a/gdb/utils.c
+++ b/gdb/utils.c
@@ -3433,7 +3433,7 @@ copy_bitwise (gdb_byte *dest, ULONGEST dest_offset,
 	buf |= *source << avail;
 
       buf &= (1 << nbits) - 1;
-      *dest = (*dest & (~0 << nbits)) | buf;
+      *dest = (*dest & (~0U << nbits)) | buf;
     }
 }