Re: [PATCH 24/26] ocfs2: reduce stack size with KASAN

Joe Perches
2017-03-02T23:59:13Z

On Thu, 2017-03-02 at 23:59 +0100, Arnd Bergmann wrote:
> KASAN decides that passing a pointer to _m into an extern function
> (_mlog_printk) is potentially dangerous, as that function might
> keep a reference to that pointer after it goes out of scope,
> or it might not know the correct length of the stack object pointed to.
>
> We can see from looking at the __mlog_printk() function definition
> that it's actually safe, but the compiler cannot see that when looking
> at another source file.

OK, thanks.

btw:

changing __mlog_printk can save ~11% (90+KB) of object text size
by removing __func__ and __LINE__ and using vsprintf pointer extension
%pS, __builtin_return_address(0) as it is already used in dlmmaster.

(defconfig x86-64, with ocfs2)

$ size fs/ocfs2/built-in.o*
text data bss dec hex filename
759791 111373 105688 976852 ee7d4 fs/ocfs2/built-in.o.new
852959 111373 105688 1070020 1053c4 fs/ocfs2/built-in.o.old

It's nearly the same output.

---

fs/ocfs2/cluster/masklog.c | 8 ++++----
fs/ocfs2/cluster/masklog.h | 8 +++-----
2 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/fs/ocfs2/cluster/masklog.c b/fs/ocfs2/cluster/masklog.c
index d331c2386b94..a3f080f37108 100644
--- a/fs/ocfs2/cluster/masklog.c
+++ b/fs/ocfs2/cluster/masklog.c
@@ -64,8 +64,7 @@ static ssize_t mlog_mask_store(u64 mask, const char *buf, size_t count)
return count;
}

-void __mlog_printk(const u64 *mask, const char *func, int line,
- const char *fmt, ...)
+void __mlog_printk(const u64 *mask, const char *fmt, ...)
{
struct va_format vaf;
va_list args;
@@ -90,9 +89,10 @@ void __mlog_printk(const u64 *mask, const char *func, int line,
vaf.fmt = fmt;
vaf.va = &args;

- printk("%s(%s,%u,%u):%s:%d %s%pV",
+ printk("%s(%s,%u,%u):%pS %s%pV",
level, current->comm, task_pid_nr(current),
- raw_smp_processor_id(), func, line, prefix, &vaf);
+ raw_smp_processor_id(), __builtin_return_address(0),
+ prefix, &vaf);

va_end(args);
}
diff --git a/fs/ocfs2/cluster/masklog.h b/fs/ocfs2/cluster/masklog.h
index 3c16da69605d..56ba5baf625b 100644
--- a/fs/ocfs2/cluster/masklog.h
+++ b/fs/ocfs2/cluster/masklog.h
@@ -162,9 +162,8 @@ extern struct mlog_bits mlog_and_bits, mlog_not_bits;

#endif

-__printf(4, 5) __nocapture(2)
-void __mlog_printk(const u64 *m, const char *func, int line,
- const char *fmt, ...);
+__printf(2, 3) __nocapture(2)
+void __mlog_printk(const u64 *m, const char *fmt, ...);

/*
* Testing before the __mlog_printk call lets the compiler eliminate the
@@ -174,8 +173,7 @@ void __mlog_printk(const u64 *m, const char *func, int line,
do { \
u64 _m = MLOG_MASK_PREFIX | (mask); \
if (_m & ML_ALLOWED_BITS) \
- __mlog_printk(&_m, __func__, __LINE__, fmt, \
- ##__VA_ARGS__); \
+ __mlog_printk(&_m, fmt, ##__VA_ARGS__); \
} while (0)

#define mlog_errno(st) ({ \