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From: Marcin Slusarz on 27 Jul 2010 19:10
When userspace code writes non-new-line-terminated string to trace_marker
file, write handler appends new-line and returns number of bytes written
to trace buffer, so
write(fd, "abc", 3) will return 4

That's unexpected and unfortunately it confuses glibc's fprintf function.

Example:
int main() {
fprintf(stderr, "abc");
return 0;
}

results in infinite loop:
write(fd, "abc", 3) = 4
write(fd, "", 1) = 0
write(fd, "", 1) = 0
write(fd, "", 1) = 0
write(fd, "", 1) = 0
write(fd, "", 1) = 0
write(fd, "", 1) = 0
write(fd, "", 1) = 0
(...)

....and kernel trace buffer full of empty markers.

Fix it by sanitizing write return value.

Signed-off-by: Marcin Slusarz <marcin.slusarz(a)gmail.com>
Cc: Steven Rostedt <rostedt(a)goodmis.org>
Cc: Frederic Weisbecker <fweisbec(a)gmail.com>
Cc: Ingo Molnar <mingo(a)redhat.com>
---
kernel/trace/trace.c | 11 ++++++++---
1 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 02e04c8..6b39aa8 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3499,6 +3499,7 @@ tracing_mark_write(struct file *filp, const char __user *ubuf,
size_t cnt, loff_t *fpos)
{
char *buf;
+ size_t written;

if (tracing_disabled)
return -EINVAL;
@@ -3520,11 +3521,15 @@ tracing_mark_write(struct file *filp, const char __user *ubuf,
} else
buf[cnt] = '\0';

- cnt = mark_printk("%s", buf);
+ written = mark_printk("%s", buf);
kfree(buf);
- *fpos += cnt;
+ *fpos += written;

- return cnt;
+ /* don't tell userspace we wrote more - it might confuse them */
+ if (written > cnt)
+ written = cnt;
+
+ return written;
}

static int tracing_clock_show(struct seq_file *m, void *v)
--
1.7.1.1

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Prev: [RFC][PATCH] PCI / PCIe: Ask BIOS for control of all native services at once (v2)
Next: mm: Check NULL pointer Dereference in mm/filemap.c