Re: potential buffer overflow in version 1.27

[Antonio Diaz Diaz]

Hello Michal,

Thank you for reporting this.

Michal Ambroz wrote:
> during compilation of ddrescue 1.27 suggests couple of places which
> potentially might result in some buffer overflow.

Not a "couple of places". Just one at main_common.cc:278, included from 
main.cc:162 and from ddrescuelog.cc:107.

> main_common.cc:278:28: warning: '%lld' directive output may be truncated
> writing between 1 and 20 bytes into a region of size 16 [-Wformat-truncation]
>
>    278 |   snprintf( buf, bufsize, "%lld %s", num, p );
>        |                            ^~~~
>
> main_common.cc:278:27: note: directive argument in the range
> [-9223372036854775807, 9223372036854775807]

This is neither a buffer overflow nor a format truncation. It is a false 
warning because the absolute value of 'num' can't be larger than six digits 
with a 64-bit 'long long'.

It seems that GCC should either become more clever and check the actual (not 
the potential) range, or better yet, revert to the old behavior of assuming 
that if the programmer uses snprintf with a buffer of a given size it is 
because he wants that size.

Or are we supposed to waste memory allocating a buffer larger than 
needed/wanted just to shut up the compiler?

BTW, just one month ago, this same warning in another function was 
reported[1]. I "fixed" that by increasing the size of the buffer, but now 
I'm questioning myself if it is wise to increase the size of all buffers 
just to shut up current compilers.

[1] http://lists.gnu.org/archive/html/bug-ddrescue/2022-12/msg00002.html

Best regards,
Antonio.