Re: [PATCH v3 09/20] kbuild: implement and use depcheck to check dependency timestamps

[Kees Cook <kees@kernel.org>]

On Thu, Sep 17, 2026 at 05:06:19PM +0100, Lorenzo Stoakes (ARM) wrote:
> This can be done faster in C, so implement scripts/basic/depcheck to do so.
> 
> It does as little work as possible, reading the .cmd files from a
> directory's targets and running stat on each dependency only a single time.

Doesn't this run the risk of missing transitive deps?

C.cmd:	C.o depends on A.c and B.c and B.h
B.cmd:	B.h depends on B.data and B.script

depcheck looks at C.o and B.h's times and is happy so it drops C.cmd file,
but then see B.script has changed compared to B.h, so it keeps B.cmd,
and then the build runs and doesn't have the C.o dep list any more,
and C.o goes unbuilt?

Also, I don't think this handles if_changed commands at all? The kbuild
rebuild condition is timestamps plus if_changed's command-line check.

> diff --git a/scripts/basic/depcheck.c b/scripts/basic/depcheck.c
> new file mode 100644
> index 000000000000..622101c33650
> --- /dev/null
> +++ b/scripts/basic/depcheck.c
> @@ -0,0 +1,441 @@
> [...]
> +/* What fixdep writes, see above. */
> +#define DEPS_PREFIX		"deps_"
> +#define DEPS_RULE_PREFIX	"$(" DEPS_PREFIX
> +#define RULE_SUFFIX		":"
> +#define LINE_CONTINUATION	" \\"
> +#define WILDCARD_OPEN		"$(wildcard "
> +#define WILDCARD_CLOSE		")"
> +#define CMD_SUFFIX		".cmd"
> [...]
> +static bool is_blank(char chr)
> [...]
> +static bool str_ends_with(const char *str, const char *suffix)
> [...]
> +static bool line_starts_with(const struct line *line, const char *prefix)
> [...]
> +static bool line_ends_with(const struct line *line, const char *suffix)
> [...]
> +static bool line_is_blank(const struct line *line)
> [...]
> +static bool line_is_continued(const struct line *line)
> [...]
> +static void line_strip_continuation(struct line *line)
> [...]
> +static void line_trim(struct line *line)
> [...]
> +static bool next_line(const char **pos, const char *end, struct line *line)

There is a lot of sting handling in here. I know you're going for speed,
but it just feels like a python script doing all this would be much more
readable without wrecking speed compared to C much. Yes, C will win out,
but the maintainability of this helper does not fill me with joy. :P

> [...]
> +static const struct dep *lookup_dep(const char *path)
> +{
> +	const unsigned int key = hash_str(path);
> +	struct dep *dep;
> +	struct stat st;
> +
> +	hash_for_each_possible(dep_table, dep, hnode, key) {
> +		if (!strcmp(dep->path, path))
> +			return dep;
> +	}
> +
> +	dep = xmalloc(sizeof(*dep) + strlen(path) + 1);
> +	strcpy(dep->path, path);
> +	dep->exists = !stat(path, &st);
> +	if (dep->exists)
> +		dep->mtime = st.st_mtim;

I realize you only check "mtime" after an "exists" check, but I bristle
at leaving a time-stamp uninitialized. Can we just add an "else
dep->mtime = 0" here?

-Kees

-- 
Kees Cook