[PATCH 1/2] tomoyo: use vsnprintf() properly

[PATCH 1/2] tomoyo: use vsnprintf() properly

[Al Viro]

Idiomatic way to find how much space sprintf output would take is
	len = snprintf(NULL, 0, ...) + 1;
Once upon a time there'd been libc implementations that blew chunks
on that and somebody had come up with the following "cute" trick:
	len = snprintf((char *) &len, 1, ...) + 1;
for doing the same.  However, that's unidiomatic, harder to follow
*and* any such libc implementation would violate both C99 and POSIX
(since 2001).
	IOW, this kludge is best buried along with such libc implementations,
nevermind getting cargo-culted into newer code.  Our vsnprintf() does not
suffer that braindamage, TYVM.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
 security/tomoyo/audit.c  | 2 +-
 security/tomoyo/common.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/security/tomoyo/audit.c b/security/tomoyo/audit.c
index 023bedd9dfa3..7cf8fdbb29bf 100644
--- a/security/tomoyo/audit.c
+++ b/security/tomoyo/audit.c
@@ -423,7 +423,7 @@ void tomoyo_write_log(struct tomoyo_request_info *r, const char *fmt, ...)
 	int len;
 
 	va_start(args, fmt);
-	len = vsnprintf((char *) &len, 1, fmt, args) + 1;
+	len = vsnprintf(NULL, 0, fmt, args) + 1;
 	va_end(args);
 	va_start(args, fmt);
 	tomoyo_write_log2(r, len, fmt, args);
diff --git a/security/tomoyo/common.c b/security/tomoyo/common.c
index ff17abc96e5c..f4cd9b58b205 100644
--- a/security/tomoyo/common.c
+++ b/security/tomoyo/common.c
@@ -2057,7 +2057,7 @@ int tomoyo_supervisor(struct tomoyo_request_info *r, const char *fmt, ...)
 	bool quota_exceeded = false;
 
 	va_start(args, fmt);
-	len = vsnprintf((char *) &len, 1, fmt, args) + 1;
+	len = vsnprintf(NULL, 0, fmt, args) + 1;
 	va_end(args);
 	/* Write /sys/kernel/security/tomoyo/audit. */
 	va_start(args, fmt);
-- 
2.30.2

Re: [PATCH 1/2] tomoyo: use vsnprintf() properly

[Tetsuo Handa]

On 2022/08/21 5:25, Al Viro wrote:
> Idiomatic way to find how much space sprintf output would take is
> 	len = snprintf(NULL, 0, ...) + 1;
> Once upon a time there'd been libc implementations that blew chunks
> on that and somebody had come up with the following "cute" trick:
> 	len = snprintf((char *) &len, 1, ...) + 1;
> for doing the same.  However, that's unidiomatic, harder to follow
> *and* any such libc implementation would violate both C99 and POSIX
> (since 2001).
> 	IOW, this kludge is best buried along with such libc implementations,
> nevermind getting cargo-culted into newer code.  Our vsnprintf() does not
> suffer that braindamage, TYVM.
> 
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Thank you. You can send this change via your tree if you like.

Acked-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>