Re: [XEN PATCH v2 1/3] libxl: Fix nul-termination of the return value of libxl_xen_console_read_line()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Aug 23, 2024 at 06:05:03PM +0100, Javi Merino wrote:
> When built with ASAN, "xl dmesg" crashes in the "printf("%s", line)"
> call in main_dmesg().  ASAN reports a heap buffer overflow: an
> off-by-one access to cr->buffer.
> 
> The readconsole sysctl copies up to count characters into the buffer,
> but it does not add a null character at the end.  Despite the
> documentation of libxl_xen_console_read_line(), line_r is not
> nul-terminated if 16384 characters were copied to the buffer.
> 
> Fix this by asking xc_readconsolering() to fill the buffer up to size
> - 1.  As the number of characters in the buffer is only needed in
> libxl_xen_console_read_line(), make it a local variable there instead
> of part of the libxl__xen_console_reader struct.

Instead of playing games with the buffer size in order to add an extra
NUL character, we could possibly use libxl_write_exactly(ctx,
STDOUT_FILENO,...) in main_dmesg(), using cr->count as the buffer
length?

Regards, Roger.