[PATCH] uefi: uefivariable: fix build error on older compilers

[Colin King]

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

Curiously, we are getting the following error on older versions
of GCC:

cc1: warnings being treated as errors
uefi/uefirtvariable/uefirtvariable.c: In function 'getnextvariable_test2':
uefi/uefirtvariable/uefirtvariable.c:352:5: error: array subscript is above array bounds

..and it only happens on 32 bit builds.

Looking at the compiled object code, it seems that:

c = variablename[(len / sizeof(c)) - 1];

produces an add of 0xffffffff todo the subtraction of -1
which triggers the array subscript array bounds warning, where as:

uint64_t i = (len / sizeof(c)) - 1;

c = variablename[i];

produces a subtraction of 1 and everthing is fine.

This patch is a workaround.  I've hand checked the result, and it
produces the code we require, so I think the original code was OK
but GCC was being overly zealous.

Signed-off-by: Colin Ian King <colin.king at canonical.com>
---
 src/uefi/uefirtvariable/uefirtvariable.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/uefi/uefirtvariable/uefirtvariable.c b/src/uefi/uefirtvariable/uefirtvariable.c
index 7242afd..c5caa33 100644
--- a/src/uefi/uefirtvariable/uefirtvariable.c
+++ b/src/uefi/uefirtvariable/uefirtvariable.c
@@ -349,7 +349,9 @@ static bool strlen_valid(uint16_t *variablename, uint64_t variablenamesize)
 	uint16_t c;
 
 	for (len = 2; len <= variablenamesize; len += sizeof(c)) {
-		c = variablename[(len / sizeof(c)) - 1];
+		uint64_t i = (len / sizeof(c)) - 1;
+
+		c = variablename[i];
 		if (!c)
 			break;
 	}
-- 
1.8.1.2