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Subject[for-next][PATCH 6/8] tracing: Replace kmap with copy_from_user() in trace_marker writing
From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>

Instead of using get_user_pages_fast() and kmap_atomic() when writing
to the trace_marker file, just allocate enough space on the ring buffer
directly, and write into it via copy_from_user().

Writing into the trace_marker file use to allocate a temporary buffer
to perform the copy_from_user(), as we didn't want to write into the
ring buffer if the copy failed. But as a trace_marker write is suppose
to be extremely fast, and allocating memory causes other tracepoints to
trigger, Peter Zijlstra suggested using get_user_pages_fast() and
kmap_atomic() to keep the user space pages in memory and reading it
directly. But Henrik Austad had issues with this because it required taking
the mm->mmap_sem and causing long delays with the write.

Instead, just allocate the space in the ring buffer and use
copy_from_user() directly. If it faults, return -EFAULT and write
"<faulted>" into the ring buffer.

Link: http://lkml.kernel.org/r/20161208124018.72dd0f86@gandalf.local.home

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Henrik Austad <henrik@austad.us>
Cc: Peter Zijlstra <peterz@infradead.org>
Updates: d696b58ca2c3ca "tracing: Do not allocate buffer for trace_marker"
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
kernel/trace/trace.c | 139 ++++++++++++++-------------------------------------
1 file changed, 37 insertions(+), 102 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 60416bf7c591..6f420d7b703b 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -5738,61 +5738,6 @@ tracing_free_buffer_release(struct inode *inode, struct file *filp)
return 0;
}

-static inline int lock_user_pages(const char __user *ubuf, size_t cnt,
- struct page **pages, void **map_page,
- int *offset)
-{
- unsigned long addr = (unsigned long)ubuf;
- int nr_pages = 1;
- int ret;
- int i;
-
- /*
- * Userspace is injecting traces into the kernel trace buffer.
- * We want to be as non intrusive as possible.
- * To do so, we do not want to allocate any special buffers
- * or take any locks, but instead write the userspace data
- * straight into the ring buffer.
- *
- * First we need to pin the userspace buffer into memory,
- * which, most likely it is, because it just referenced it.
- * But there's no guarantee that it is. By using get_user_pages_fast()
- * and kmap_atomic/kunmap_atomic() we can get access to the
- * pages directly. We then write the data directly into the
- * ring buffer.
- */
-
- /* check if we cross pages */
- if ((addr & PAGE_MASK) != ((addr + cnt) & PAGE_MASK))
- nr_pages = 2;
-
- *offset = addr & (PAGE_SIZE - 1);
- addr &= PAGE_MASK;
-
- ret = get_user_pages_fast(addr, nr_pages, 0, pages);
- if (ret < nr_pages) {
- while (--ret >= 0)
- put_page(pages[ret]);
- return -EFAULT;
- }
-
- for (i = 0; i < nr_pages; i++)
- map_page[i] = kmap_atomic(pages[i]);
-
- return nr_pages;
-}
-
-static inline void unlock_user_pages(struct page **pages,
- void **map_page, int nr_pages)
-{
- int i;
-
- for (i = nr_pages - 1; i >= 0; i--) {
- kunmap_atomic(map_page[i]);
- put_page(pages[i]);
- }
-}
-
static ssize_t
tracing_mark_write(struct file *filp, const char __user *ubuf,
size_t cnt, loff_t *fpos)
@@ -5802,14 +5747,14 @@ tracing_mark_write(struct file *filp, const char __user *ubuf,
struct ring_buffer *buffer;
struct print_entry *entry;
unsigned long irq_flags;
- struct page *pages[2];
- void *map_page[2];
- int nr_pages = 1;
+ const char faulted[] = "<faulted>";
ssize_t written;
- int offset;
int size;
int len;

+/* Used in tracing_mark_raw_write() as well */
+#define FAULTED_SIZE (sizeof(faulted) - 1) /* '\0' is already accounted for */
+
if (tracing_disabled)
return -EINVAL;

@@ -5821,30 +5766,31 @@ tracing_mark_write(struct file *filp, const char __user *ubuf,

BUILD_BUG_ON(TRACE_BUF_SIZE >= PAGE_SIZE);

- nr_pages = lock_user_pages(ubuf, cnt, pages, map_page, &offset);
- if (nr_pages < 0)
- return nr_pages;
-
local_save_flags(irq_flags);
- size = sizeof(*entry) + cnt + 2; /* possible \n added */
+ size = sizeof(*entry) + cnt + 2; /* add '\0' and possible '\n' */
+
+ /* If less than "<faulted>", then make sure we can still add that */
+ if (cnt < FAULTED_SIZE)
+ size += FAULTED_SIZE - cnt;
+
buffer = tr->trace_buffer.buffer;
event = __trace_buffer_lock_reserve(buffer, TRACE_PRINT, size,
irq_flags, preempt_count());
- if (!event) {
+ if (unlikely(!event))
/* Ring buffer disabled, return as if not open for write */
- written = -EBADF;
- goto out_unlock;
- }
+ return -EBADF;

entry = ring_buffer_event_data(event);
entry->ip = _THIS_IP_;

- if (nr_pages == 2) {
- len = PAGE_SIZE - offset;
- memcpy(&entry->buf, map_page[0] + offset, len);
- memcpy(&entry->buf[len], map_page[1], cnt - len);
+ len = __copy_from_user_inatomic(&entry->buf, ubuf, cnt);
+ if (len) {
+ memcpy(&entry->buf, faulted, FAULTED_SIZE);
+ cnt = FAULTED_SIZE;
+ written = -EFAULT;
} else
- memcpy(&entry->buf, map_page[0] + offset, cnt);
+ written = cnt;
+ len = cnt;

if (entry->buf[cnt - 1] != '\n') {
entry->buf[cnt] = '\n';
@@ -5854,12 +5800,8 @@ tracing_mark_write(struct file *filp, const char __user *ubuf,

__buffer_unlock_commit(buffer, event);

- written = cnt;
-
- *fpos += written;
-
- out_unlock:
- unlock_user_pages(pages, map_page, nr_pages);
+ if (written > 0)
+ *fpos += written;

return written;
}
@@ -5875,15 +5817,14 @@ tracing_mark_raw_write(struct file *filp, const char __user *ubuf,
struct ring_buffer_event *event;
struct ring_buffer *buffer;
struct raw_data_entry *entry;
+ const char faulted[] = "<faulted>";
unsigned long irq_flags;
- struct page *pages[2];
- void *map_page[2];
- int nr_pages = 1;
ssize_t written;
- int offset;
int size;
int len;

+#define FAULT_SIZE_ID (FAULTED_SIZE + sizeof(int))
+
if (tracing_disabled)
return -EINVAL;

@@ -5899,38 +5840,32 @@ tracing_mark_raw_write(struct file *filp, const char __user *ubuf,

BUILD_BUG_ON(TRACE_BUF_SIZE >= PAGE_SIZE);

- nr_pages = lock_user_pages(ubuf, cnt, pages, map_page, &offset);
- if (nr_pages < 0)
- return nr_pages;
-
local_save_flags(irq_flags);
size = sizeof(*entry) + cnt;
+ if (cnt < FAULT_SIZE_ID)
+ size += FAULT_SIZE_ID - cnt;
+
buffer = tr->trace_buffer.buffer;
event = __trace_buffer_lock_reserve(buffer, TRACE_RAW_DATA, size,
irq_flags, preempt_count());
- if (!event) {
+ if (!event)
/* Ring buffer disabled, return as if not open for write */
- written = -EBADF;
- goto out_unlock;
- }
+ return -EBADF;

entry = ring_buffer_event_data(event);

- if (nr_pages == 2) {
- len = PAGE_SIZE - offset;
- memcpy(&entry->id, map_page[0] + offset, len);
- memcpy(((char *)&entry->id) + len, map_page[1], cnt - len);
+ len = __copy_from_user_inatomic(&entry->id, ubuf, cnt);
+ if (len) {
+ entry->id = -1;
+ memcpy(&entry->buf, faulted, FAULTED_SIZE);
+ written = -EFAULT;
} else
- memcpy(&entry->id, map_page[0] + offset, cnt);
+ written = cnt;

__buffer_unlock_commit(buffer, event);

- written = cnt;
-
- *fpos += written;
-
- out_unlock:
- unlock_user_pages(pages, map_page, nr_pages);
+ if (written > 0)
+ *fpos += written;

return written;
}
--
2.10.2

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