From: Steven Rostedt <rost...@goodmis.org>
A discussion came up in the trace triggers thread about converting a
bunch of:
strncmp(str, "const", sizeof("const") - 1)
use cases into a helper macro. It started with:
strncmp(str, const, sizeof(const) - 1)
But then Joe Perches mentioned that if a const is not used, the
sizeof() will be the size of a pointer, which can be bad. And that
gcc will optimize strlen("const") into "sizeof("const") - 1".
Thinking about this more, a quick grep in the kernel tree found several
(thousands!) of cases that use this construct. A quick grep also
revealed that there's probably several bugs in that use case. Some are
that people forgot the "- 1" (which I found) and others could be that
the constant for the sizeof is different than the constant (although, I
haven't found any of those, but I also didn't look hard).
I figured the best thing to do is to create a helper macro and place it
into include/linux/string.h. And go around and fix all the open coded
versions of it later.
I plan on only applying this patch and updating the tracing hooks for
this merge window. And perhaps use it to fix some of the bugs that were
found.
I was going to just use:
strncmp(str, prefix, strlen(prefix))
but then realized that "prefix" is used twice, and will break if
someone does something like:
strncmp_prefix(str, ptr++);
So instead I check with __builtin_constant_p() to see if the second
parameter is truly a constant, which I use sizeof() anyway (why bother
gcc to optimize it, if we already know it's a constant), and copy the
parameter into a local variable and use that local variable for the non
constant part (with strlen).
Link:
http://lkml.kernel.org/r/e3e754f2bd18e56eaa8baf79bee619316ebf4cfc.1545161087.git.tom.zanu...@linux.intel.com
Link: http://lkml.kernel.org/r/20181219211615.2298e...@gandalf.local.home
Cc: Joe Perches <j...@perches.com>
Cc: Tom Zanussi <zanu...@kernel.org>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org>
Signed-off-by: Steven Rostedt (VMware) <rost...@goodmis.org>
---
include/linux/string.h | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
diff --git a/include/linux/string.h b/include/linux/string.h
index 27d0482e5e05..a998a53333ef 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -14,6 +14,28 @@ extern void *memdup_user(const void __user *, size_t);
extern void *vmemdup_user(const void __user *, size_t);
extern void *memdup_user_nul(const void __user *, size_t);
+/*
+ * A common way to test a prefix of a string is to do:
+ * strncmp(str, prefix, sizeof(prefix) - 1)
+ *
+ * But this can lead to bugs due to typos, or if prefix is a pointer
+ * and not a constant. Instead use strncmp_prefix().
+ */
+#define strncmp_prefix(str, prefix) \
+ ({ \
+ int ____strcmp_prefix_ret____; \
+ if (__builtin_constant_p(&prefix)) { \
+ ____strcmp_prefix_ret____ = \
+ strncmp(str, prefix, sizeof(prefix) - 1); \
+ } else { \
+ typeof(prefix) ____strcmp_prefix____ = prefix; \
+ ____strcmp_prefix_ret____ = \
+ strncmp(str, ____strcmp_prefix____, \
+ strlen(____strcmp_prefix____)); \
+ } \
+ ____strcmp_prefix_ret____; \
+ })
+
/*
* Include machine specific inline routines
*/
--
2.19.2
