Re: [PATCH 2/3] printk: Add %ptc to safely print a task's comm

Joe Perches
2011-05-11T00:51:34Z

On Tue, 2011-05-10 at 17:23 -0700, John Stultz wrote:
> Acessing task->comm requires proper locking. However in the past
> access to current->comm could be done without locking. This
> is no longer the case, so all comm access needs to be done
> while holding the comm_lock.
>
> In my attempt to clean up unprotected comm access, I've noticed
> most comm access is done for printk output. To simpify correct
> locking in these cases, I've introduced a new %ptc format,
> which will safely print the corresponding task's comm.

Hi John.

Couple of tyops for Accessing and simplify in your commit message
and a few comments on the patch.

Could misuse of %ptc (not using current) cause system lockup?

> Example use:
> printk("%ptc: unaligned epc - sending SIGBUS.\n", current);

> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index bc0ac6b..b9c97b8 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -797,6 +797,26 @@ char *uuid_string(char *buf, char *end, const u8 *addr,
> return string(buf, end, uuid, spec);
> }
>
> +static noinline_for_stack
> +char *task_comm_string(char *buf, char *end, u8 *addr,
> + struct printf_spec spec, const char *fmt)

addr should be void * not u8 *

> +{
> + struct task_struct *tsk = (struct task_struct *) addr;

no cast.

Maybe it'd be better to use current inside this routine and not
pass the pointer at all.

static noinline_for_stack
char *task_comm_string(char *buf, char *end,
struct printf_spec spec, const char *fmt)

> + char *ret;
> + unsigned long seq;
> +
> + do {
> + seq = read_seqbegin(&tsk->comm_lock);
> +
> + ret = string(buf, end, tsk->comm, spec);
> +
> + } while (read_seqretry(&tsk->comm_lock, seq));

> @@ -864,6 +884,12 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
> }
>
> switch (*fmt) {
> + case 't':
> + switch (fmt[1]) {
> + case 'c':
> + return task_comm_string(buf, end, ptr, spec, fmt);

maybe
return task_comm_string(buf, end, spec, fmt);