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Breno Leitao
07e82e2825
[ Upstream commitdec4d8118c] xbc_snprint_cmdline() is meant to be called twice: first with buf=NULL, size=0 to probe the rendered length, then with a real buffer to fill it (the standard snprintf() two-pass pattern). The probe call makes the function compute "buf + size" (NULL + 0) and, on every iteration, advance "buf += ret" from that NULL base and pass the result back into snprintf(). Pointer arithmetic on a NULL pointer is undefined behavior. It is harmless in the in-kernel callers today, but the follow-up patches run this same code in the userspace tools/bootconfig parser at kernel build time, where host UBSan / FORTIFY_SOURCE abort the build. Track a running written length (size_t) instead of mutating @buf, and only form "buf + len" when @buf is non-NULL. snprintf(NULL, 0, ...) is itself well defined and returns the would-be length, so the two-pass "probe then fill" usage returns identical byte counts. Link: https://lore.kernel.org/all/20260626-bootconfig_using_tools-v7-1-24ab72139c29@debian.org/ Fixes:51887d03ac("bootconfig: init: Allow admin to use bootconfig for kernel command line") Cc: stable@vger.kernel.org Signed-off-by: Breno Leitao <leitao@debian.org> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org> Signed-off-by: Sasha Levin <sashal@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>