Re: [PATCH] lib/bootconfig: fix undefined behavior involving NULL pointer arithmetic

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon, 29 Jun 2026 14:53:05 +0100
Bradley Morgan <include@grrlz.net> wrote:

> On 29 June 2026 14:41:37 BST, Breno Leitao <leitao@debian.org> wrote:
> >On Sun, Jun 28, 2026 at 11:56:16AM +0000, Bradley Morgan wrote:
> >> When xbc_snprint_cmdline() is called during the size-probing phase
> >> (with buf = NULL and size = 0), the function computes the end pointer
> >> as 'buf + size' (NULL + 0) and repeatedly advances the pointer via
> >> 'buf += ret'.
> >> 
> >> Under the C standard, performing pointer arithmetic on a NULL pointer is
> >> undefined behavior. While harmless inside the kernel, this code is also
> >> compiled into the userspace host tool 'tools/bootconfig', where host
> >> compilers with UBSan or FORTIFY_SOURCE enabled abort the build when they
> >> detect NULL pointer arithmetic.
> >> 
> >> Fix this by tracking the running written length as an integer offset
> >> ('len') rather than advancing 'buf' directly. Only perform pointer
> >> arithmetic if 'buf' is actually non-NULL.
> >> 
> >> Fixes: 5a643e462323 ("bootconfig: move xbc_snprint_cmdline() to
> >lib/bootconfig.c")
> >
> >Isn't commit 5a643e462323 ("bootconfig: move xbc_snprint_cmdline() to
> >lib/bootconfig.c") just a code movement?
> 
> Ugh, Geminis bullcrap, you are right. I should've just manually looked
> for the fixes tag (as I always do)

Yeah, please use the latest linus kernel. (v7.2-rc1, for now)

> 
> >>  	xbc_node_for_each_key_value(root, knode, val) {
> >> @@ -439,10 +437,12 @@ int __init xbc_snprint_cmdline(char *buf, size_t
> >size, struct xbc_node *root)
> >>  
> >>  		vnode = xbc_node_get_child(knode);
> >>  		if (!vnode) {
> >> -			ret = snprintf(buf, rest(buf, end), "%s ", xbc_namebuf);
> >> +			ret = snprintf(buf ? buf + len : NULL,
> >> +				       size > len ? size - len : 0,
> >
> >Why not keeping rest() and updating it, instead of open coding it?
> >
> >Thanks for the fix.
> 
> sure I'll do V2, btw if u didn't read, gemini found and fixed this.
> As in fully. :)

Hint: for fixing an issue, please just focus on fixing the issue
and try minimizing the change for keeping backportability.

Thanks,

> 
> 
> 
> >--breno
> >
> 
> Thanks!


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>