[PATCH] fwts_safe_mem: workaround longjmp clobber warnings

[Colin King]

From: Colin Ian King <colin.king at canonical.com>

On some flavours of GCC the src to fwts_safe_memread throws up
a longjmp clobber warning:

fwts_safe_mem.c: In function ‘fwts_safe_memread’:
fwts_safe_mem.c:69:35: error: argument ‘src’ might be clobbered by ‘longjmp’
or ‘vfork’ [-Werror=clobbered]
 int fwts_safe_memread(const void *src, const size_t n)
                                   ^
cc1: all warnings being treated as errors

We either disable -Wclobber (which I don't want to do since it is useful
to catch these potential errors) or work out a case by case workaround.
The easiest fix for these is to crank down the optimisation level to zero
so that the compiler stashes the src argument onto the stack and we don't
end up with clobbering.

So.. add a bunch of macros to use a gcc attribute if it is supported
and force the function to be optimized at level 0.

Signed-off-by: Colin Ian King <colin.king at canonical.com>
---
 src/lib/include/fwts.h      | 24 ++++++++++++++++++++++++
 src/lib/src/fwts_safe_mem.c |  4 ++--
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/src/lib/include/fwts.h b/src/lib/include/fwts.h
index a56ce24f..325e4ee4 100644
--- a/src/lib/include/fwts.h
+++ b/src/lib/include/fwts.h
@@ -44,6 +44,30 @@
 #undef FWTS_HAS_UEFI
 #endif
 
+/* verision 3-tuple into integer */
+#define _VER_(major, minor, patchlevel)                 \
+	((major * 10000) + (minor * 100) + patchlevel)
+
+/* check version of GNU GCC */
+#if defined(__GNUC__) && defined(__GNUC_MINOR__)
+#if defined(__GNUC_PATCHLEVEL__)
+#define NEED_GNUC(major, minor, patchlevel)                     \
+	_VER_(major, minor, patchlevel) <= _VER_(__GNUC__, __GNUC_MINOR__, __GNUC_PATCHLEVEL__)
+#else
+#define NEED_GNUC(major, minor, patchlevel)                     \
+	_VER_(major, minor, patchlevel) <= _VER_(__GNUC__, __GNUC_MINOR__, 0)
+#endif
+#else
+#define NEED_GNUC(major, minor, patchlevel)     (0)
+#endif
+
+/* -O0 attribute support */
+#if defined(__GNUC__) && !defined(__clang__) && NEED_GNUC(4,6,0)
+#define OPTIMIZE0	__attribute__((optimize("-O0")))
+#else
+#define OPTIMIZE0
+#endif
+
 #define FWTS_UNUSED(var)	(void)var
 
 #define FWTS_JSON_DATA_PATH	DATAROOTDIR "/fwts"
diff --git a/src/lib/src/fwts_safe_mem.c b/src/lib/src/fwts_safe_mem.c
index 216ad8e2..1feb1ff2 100644
--- a/src/lib/src/fwts_safe_mem.c
+++ b/src/lib/src/fwts_safe_mem.c
@@ -46,7 +46,7 @@ static void sig_handler(int dummy)
  *	may segfault or throw a bus error if the src
  *	address is corrupt
  */
-int fwts_safe_memcpy(void *dst, const void *src, const size_t n)
+int OPTIMIZE0 fwts_safe_memcpy(void *dst, const void *src, const size_t n)
 {
 	if (sigsetjmp(jmpbuf, 1) != 0)
 		return FWTS_ERROR;
@@ -66,7 +66,7 @@ int fwts_safe_memcpy(void *dst, const void *src, const size_t n)
  *	SIGSEGV/SIGBUS errors and returns FWTS_ERROR if it is not
  *	readable or FWTS_OK if it's OK.
  */
-int fwts_safe_memread(const void *src, const size_t n)
+int OPTIMIZE0 fwts_safe_memread(const void *src, const size_t n)
 {
 	const volatile uint8_t *ptr = src;
 	const volatile uint8_t *end = ptr + n;
-- 
2.11.0