[PATCH xfsprogs-5.14.2 URGENT] libxfs: hide the drainbamaged fallthrough macro from xfslibs

Darrick J. Wong
2021-12-05T17:49:55Z

From: Darrick J. Wong <djwong@kernel.org>

Back in mid-2021, Kees and Gustavo rammed into the kernel a bunch of
static checker "improvements" that redefined '/* fallthrough */'
comments for switch statements as a macro that virtualizes either that
same comment, a do-while loop, or a compiler __attribute__. This was
necessary to work around the poor decision-making of the clang, gcc, and
C language standard authors, who collectively came up with four mutually
incompatible ways to document a lack of branching in a code flow.

Having received ZERO HELP porting this to userspace, Eric and I
foolishly dumped that crap into linux.h, which was a poor decision
because we keep forgetting that linux.h is exported as a userspace
header. This has now caused downstream regressions in Debian[1] and
will probably cause more problems in the other distros.

Move it to platform_defs.h since that's not shipped publicly and leave a
warning to anyone else who dare modify linux.h.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1000974

Fixes: df9c7d8d ("xfs: Fix fall-through warnings for Clang")
Cc: 1000974@bugs.debian.org, gustavoars@kernel.org, keescook@chromium.org
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
include/linux.h | 20 ++------------------
include/platform_defs.h.in | 21 +++++++++++++++++++++
2 files changed, 23 insertions(+), 18 deletions(-)

diff --git a/include/linux.h b/include/linux.h
index 24650228..054117aa 100644
--- a/include/linux.h
+++ b/include/linux.h
@@ -360,24 +360,8 @@ fsmap_advance(
#endif /* HAVE_MAP_SYNC */

/*
- * Add the pseudo keyword 'fallthrough' so case statement blocks
- * must end with any of these keywords:
- * break;
- * fallthrough;
- * continue;
- * goto ;
- * return [expression];
- *
- * gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#Statement-Attributes
+ * Reminder: anything added to this file will be compiled into downstream
+ * userspace projects!
*/
-#if defined __has_attribute
-# if __has_attribute(__fallthrough__)
-# define fallthrough __attribute__((__fallthrough__))
-# else
-# define fallthrough do {} while (0) /* fallthrough */
-# endif
-#else
-# define fallthrough do {} while (0) /* fallthrough */
-#endif

#endif /* __XFS_LINUX_H__ */
diff --git a/include/platform_defs.h.in b/include/platform_defs.h.in
index 7c6b3ada..6e6f26ef 100644
--- a/include/platform_defs.h.in
+++ b/include/platform_defs.h.in
@@ -113,4 +113,25 @@ static inline size_t __ab_c_size(size_t a, size_t b, size_t c)
sizeof(*(p)->member) + __must_be_array((p)->member), \
sizeof(*(p)))

+/*
+ * Add the pseudo keyword 'fallthrough' so case statement blocks
+ * must end with any of these keywords:
+ * break;
+ * fallthrough;
+ * continue;
+ * goto ;
+ * return [expression];
+ *
+ * gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#Statement-Attributes
+ */
+#if defined __has_attribute
+# if __has_attribute(__fallthrough__)
+# define fallthrough __attribute__((__fallthrough__))
+# else
+# define fallthrough do {} while (0) /* fallthrough */
+# endif
+#else
+# define fallthrough do {} while (0) /* fallthrough */
+#endif
+
#endif /* __XFS_PLATFORM_DEFS_H__ */