[PATCH v2] selinux: replace strlcat() with seq_buf in selinux_ima_collect_state()

[Ian Bridges]

In preparation for removing the deprecated strlcat() API[1], replace the
strscpy()/strlcat() chain in selinux_ima_collect_state() with a struct
seq_buf, which tracks the write position and remaining space internally.

Each field is written with seq_buf_printf() using a "=%d;" format, which
removes the open-coded "=1;"/"=0;" constants. The seven per-append
WARN_ON(rc >= buf_len) truncation checks are replaced by a single
seq_buf_has_overflowed() check after the string is built.

Link: https://github.com/KSPP/linux/issues/370 [1]
Signed-off-by: Ian Bridges <i...@fastmail.org>
---
Changed in v2: replace the v1 seq_buf_puts() pairs with seq_buf_printf()
using a "=%d;" format, which drops the open-coded "=1;"/"=0;" constants.

v1: https://lore.kernel.org/all/ajlN94VO7BYNUTAy@dev/

I didn't change the precomputation of the string size. An alternative,
which is used by other seq_buf callers (e.g. kernel/rcu/refscale.c,
mm/memcontrol.c), is to drop the precomputation and allocate an oversized
fixed buffer, relying on the seq_buf overflow check as a backstop. I'm
happy to rework the patch to adopt that alternative.

 security/selinux/ima.c | 40 +++++++++++++---------------------------
 1 file changed, 13 insertions(+), 27 deletions(-)

diff --git a/security/selinux/ima.c b/security/selinux/ima.c
index aa34da9b0aeb..cb0efa2fc1ad 100644
--- a/security/selinux/ima.c
+++ b/security/selinux/ima.c
@@ -9,6 +9,7 @@
  */
 #include <linux/vmalloc.h>
 #include <linux/ima.h>
+#include <linux/seq_buf.h>
 #include "security.h"
 #include "ima.h"
 
@@ -20,46 +21,31 @@
  */
 static char *selinux_ima_collect_state(void)
 {
-       const char *on = "=1;", *off = "=0;";
+       struct seq_buf s;
        char *buf;
-       int buf_len, len, i, rc;
+       int buf_len, suffix_len, i;
 
        buf_len = strlen("initialized=0;enforcing=0;checkreqprot=0;") + 1;
+       suffix_len = strlen("=0;");
 
-       len = strlen(on);
        for (i = 0; i < __POLICYDB_CAP_MAX; i++)
-               buf_len += strlen(selinux_policycap_names[i]) + len;
+               buf_len += strlen(selinux_policycap_names[i]) + suffix_len;
 
        buf = kzalloc(buf_len, GFP_KERNEL);
        if (!buf)
                return NULL;
 
-       rc = strscpy(buf, "initialized", buf_len);
-       WARN_ON(rc < 0);
+       seq_buf_init(&s, buf, buf_len);
 
-       rc = strlcat(buf, selinux_initialized() ? on : off, buf_len);
-       WARN_ON(rc >= buf_len);
+       seq_buf_printf(&s, "initialized=%d;enforcing=%d;checkreqprot=%d;",
+                      selinux_initialized(), enforcing_enabled(),
+                      checkreqprot_get());
 
-       rc = strlcat(buf, "enforcing", buf_len);
-       WARN_ON(rc >= buf_len);
-
-       rc = strlcat(buf, enforcing_enabled() ? on : off, buf_len);
-       WARN_ON(rc >= buf_len);
-
-       rc = strlcat(buf, "checkreqprot", buf_len);
-       WARN_ON(rc >= buf_len);
-
-       rc = strlcat(buf, checkreqprot_get() ? on : off, buf_len);
-       WARN_ON(rc >= buf_len);
-
-       for (i = 0; i < __POLICYDB_CAP_MAX; i++) {
-               rc = strlcat(buf, selinux_policycap_names[i], buf_len);
-               WARN_ON(rc >= buf_len);
+       for (i = 0; i < __POLICYDB_CAP_MAX; i++)
+               seq_buf_printf(&s, "%s=%d;", selinux_policycap_names[i],
+                              selinux_state.policycap[i]);
 
-               rc = strlcat(buf, selinux_state.policycap[i] ? on : off,
-                       buf_len);
-               WARN_ON(rc >= buf_len);
-       }
+       WARN_ON(seq_buf_has_overflowed(&s));
 
        return buf;
 }
-- 
2.47.3