[PATCH] uapi: change TRACE_MMAP_IOCTL_GET_READER to avoid collision with TCGETS

[PATCH] uapi: change TRACE_MMAP_IOCTL_GET_READER to avoid collision with TCGETS

[Dmitry V. Levin]

The number that was initially chosen for TRACE_MMAP_IOCTL_GET_READER,
unfortunately, collides with TCGETS on most of architectures.

For example, this is how strace output would look like when
support for this value of TRACE_MMAP_IOCTL_GET_READER is added:

$ strace -e ioctl stty
ioctl(0, TCGETS or TRACE_MMAP_IOCTL_GET_READER, {c_iflag=ICRNL|IXON, c_oflag=NL0|CR0|TAB0|BS0|VT0|FF0|OPOST|ONLCR,
+c_cflag=B38400|CS8|CREAD, c_lflag=ISIG|ICANON|ECHO|ECHOE|ECHOK|IEXTEN|ECHOCTL|ECHOKE, ...}) = 0

Even though ioctl numbers are inherently not unique, TCGETS
is a very traditional one, so let's change the value of
TRACE_MMAP_IOCTL_GET_READER a bit to avoid this collision.

Given that _IO('T', 0x1) is _IOC(_IOC_NONE, 'T', 0x1, 0),
something like _IOC(_IOC_NONE, 'T', 0x1, 0x1) should be OK.

Fixes: cf9f0f7c4c5bb ("tracing: Allow user-space mapping of the ring-buffer")
Signed-off-by: Dmitry V. Levin <ldv@strace.io>
---
 include/uapi/linux/trace_mmap.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/uapi/linux/trace_mmap.h b/include/uapi/linux/trace_mmap.h
index bd1066754220..cb858f1b8367 100644
--- a/include/uapi/linux/trace_mmap.h
+++ b/include/uapi/linux/trace_mmap.h
@@ -43,6 +43,6 @@ struct trace_buffer_meta {
 	__u64	Reserved2;
 };
 
-#define TRACE_MMAP_IOCTL_GET_READER		_IO('T', 0x1)
+#define TRACE_MMAP_IOCTL_GET_READER		_IOC(_IOC_NONE, 'T', 0x1, 0x1)
 
 #endif /* _TRACE_MMAP_H_ */
-- 
ldv