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Subject: Re: [PATCH] Undefined behavior reported in copy_bitwise
To: krokus <nomadbyte@gmail.com>, gdb-patches@sourceware.org
References: <CAMCMYR5=V6WBN-AZw4R8EAVxkRt4T62G0JCARAaCc77PiwF7LQ@mail.gmail.com>
From: Simon Marchi <simark@simark.ca>
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Date: Fri, 10 Apr 2020 10:59:00 -0400
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On 2020-04-09 7:50 p.m., krokus via Gdb-patches wrote:
> 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.
> 
> patch -p1:
> ````````
> --- gdb/utils.c    2020-04-09 18:41:03.339065535 -0500
> +++ gdb/utils.c    2020-04-09 18:41:24.427064851 -0500
> @@ -3429,7 +3429,7 @@
>      buf |= *source << avail;
> 
>        buf &= (1 << nbits) - 1;
> -      *dest = (*dest & (~0 << nbits)) | buf;
> +      *dest = (*dest & (~0U << nbits)) | buf;
>      }
>  }
> ````````
> 

Thanks.  I'll try to reproduce it, but at first sight it seems fine.

Could you please provide a ChangeLog entry for this, with the name and
email address you'd like to have in there?  See the gdb/ChangeLog file
for examples.

Simon