Re: [PATCH] i386: Fix GCC warning with snprintf when HAX is enabled

[Richard Henderson]

On 3/1/20 8:35 AM, Julio Faracco wrote:
> When HAX is enabled (--enable-hax), GCC 9.2.1 reports issues with
> snprintf(). This commit is checking if snprintf returns an error. This
> is a simple way to avoid this warnings.
> 
> For more details, one example of warning:
>   CC      i386-softmmu/target/i386/hax-posix.o
> qemu/target/i386/hax-posix.c: In function ‘hax_host_open_vm’:
> qemu/target/i386/hax-posix.c:124:56: error: ‘%02d’ directive output may be
> truncated writing between 2 and 11 bytes into a region of size 3
> [-Werror=format-truncation=]
>   124 |     snprintf(name, sizeof HAX_VM_DEVFS, "/dev/hax_vm/vm%02d", vm_id);
>       |                                                        ^~~~
> qemu/target/i386/hax-posix.c:124:41: note: directive argument in the range
> [-2147483648, 64]


The code bounds vm_id from the top (64) but not the bottom.  The compiler has
(correctly) determined that negative values will overflow your buffer.

I suggest either an assert vs negative values as a local change, or to change
the type of vm_id (universally, if possible) to an unsigned type.


r~