lib/vsprintf: add %pT format specifier

Since task_struct->comm can be modified by other threads while the current
thread is reading it, it is recommended to use get_task_comm() for reading
it.

However, since get_task_comm() holds task_struct->alloc_lock spinlock,
some users cannot use get_task_comm().  Also, a lot of users are directly
reading from task_struct->comm even if they can use get_task_comm().  Such
users might obtain inconsistent result.

This patch introduces %pT format specifier for printing task_struct->comm.
Currently %pT does not provide consistency.  I'm planning to change to
use RCU in the future.  By using RCU, the comm name read from
task_struct->comm will be guaranteed to be consistent.  But before
modifying set_task_comm() to use RCU, we need to kill direct ->comm users
who do not use get_task_comm().

An example for converting direct ->comm users is shown below.  Since many
debug printings use p == current, you can pass NULL instead of p if p ==
current.

  pr_info("comm=%s\n", p->comm);       => pr_info("comm=%pT\n", p);
  pr_info("comm=%s\n", current->comm); => pr_info("comm=%pT\n", NULL);

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 25 insertions, 1 deletions

diff --git a/Documentation/printk-formats.txt b/Documentation/printk-formats.txt
index 6f4eb322ffaf..94459b42e0ab 100644
--- a/Documentation/printk-formats.txt
+++ b/Documentation/printk-formats.txt
@@ -184,6 +184,12 @@ dentry names:
 	equivalent of %s dentry->d_name.name we used to use, %pd<n> prints
 	n last components.  %pD does the same thing for struct file.
 
+task_struct comm name:
+
+        %pT
+
+        For printing task_struct->comm.
+
 struct va_format:
 
 	%pV
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 0648291cdafe..e30d885d9631 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1183,6 +1183,21 @@ char *address_val(char *buf, char *end, const void *addr,
 	return number(buf, end, num, spec);
 }
 
+static noinline_for_stack
+char *comm_name(char *buf, char *end, struct task_struct *tsk,
+		struct printf_spec spec, const char *fmt)
+{
+	char name[TASK_COMM_LEN];
+
+	/* Caller can pass NULL instead of current. */
+	if (!tsk)
+		tsk = current;
+	/* Not using get_task_comm() in case I'm in IRQ context. */
+	memcpy(name, tsk->comm, TASK_COMM_LEN);
+	name[sizeof(name) - 1] = '\0';
+	return string(buf, end, name, spec);
+}
+
 int kptr_restrict __read_mostly;
 
 /*
@@ -1250,6 +1265,7 @@ int kptr_restrict __read_mostly;
  *           (default assumed to be phys_addr_t, passed by reference)
  * - 'd[234]' For a dentry name (optionally 2-4 last components)
  * - 'D[234]' Same as 'd' but for a struct file
+ * - 'T' task_struct->comm
  *
  * Note: The difference between 'S' and 'F' is that on ia64 and ppc64
  * function pointers are really function descriptors, which contain a
@@ -1261,7 +1277,7 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
 {
 	int default_width = 2 * sizeof(void *) + (spec.flags & SPECIAL ? 2 : 0);
 
-	if (!ptr && *fmt != 'K') {
+	if (!ptr && *fmt != 'K' && *fmt != 'T') {
 		/*
 		 * Print (null) with the same width as a pointer so it makes
 		 * tabular output look nice.
@@ -1389,6 +1405,8 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
 		return dentry_name(buf, end,
 				   ((const struct file *)ptr)->f_path.dentry,
 				   spec, fmt);
+	case 'T':
+		return comm_name(buf, end, ptr, spec, fmt);
 	}
 	spec.flags |= SMALL;
 	if (spec.field_width == -1) {