mtd: Replace zero-length array with flexible-array

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] KSPP#21
[3] commit 7649773 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <[email protected]>
Acked-by: Miquel Raynal <[email protected]>
Signed-off-by: Vignesh Raghavendra <[email protected]>
Link: https://lore.kernel.org/r/20200507190033.GA15215@embeddedor

Author: GustavoARSilva

include/linux/mtd/cfi.h

@@ -138,7 +138,7 @@ struct cfi_ident {
 	uint16_t InterfaceDesc;
 	uint16_t MaxBufWriteSize;
 	uint8_t  NumEraseRegions;
-	uint32_t EraseRegionInfo[0]; /* Not host ordered */
+	uint32_t EraseRegionInfo[]; /* Not host ordered */
 } __packed;
 
 /* Extended Query Structure for both PRI and ALT */
@@ -165,7 +165,7 @@ struct cfi_pri_intelext {
 	uint16_t ProtRegAddr;
 	uint8_t  FactProtRegSize;
 	uint8_t  UserProtRegSize;
-	uint8_t  extra[0];
+	uint8_t  extra[];
 } __packed;
 
 struct cfi_intelext_otpinfo {
@@ -286,7 +286,7 @@ struct cfi_private {
 	map_word sector_erase_cmd;
 	unsigned long chipshift; /* Because they're of the same type */
 	const char *im_name;	 /* inter_module name for cmdset_setup */
-	struct flchip chips[0];  /* per-chip data structure for each chip */
+	struct flchip chips[];  /* per-chip data structure for each chip */
 };
 
 uint32_t cfi_build_cmd_addr(uint32_t cmd_ofs,

include/linux/mtd/qinfo.h

@@ -24,7 +24,7 @@ struct lpddr_private {
 	struct qinfo_chip *qinfo;
 	int numchips;
 	unsigned long chipshift;
-	struct flchip chips[0];
+	struct flchip chips[];
 };
 
 /* qinfo_query_info structure contains request information for