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mtd: Replace zero-length array with flexible-array

PATCH Applied Gustavo A. R. Silva 3 participants 3 messages May 7, 2020 – May 26, 2020 Memory technology devices (MTD)

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Gustavo A. R. Silva

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PATCH 1/1 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] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by Gustavo A. R. Silva <[email protected]>
---
 include/linux/mtd/cfi.h   |    6 +++---
 include/linux/mtd/qinfo.h |    2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)
include/linux/mtd/cfi.h +3 -3
index c98a21108688..fd1ecb821106 100644
@@ -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 +1 -1
index df5b9fddea16..2e3f43788d48 100644
@@ -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
maintainer Acked-by on v1 1/1
#1 lore raw

Hi Gustavo,

"Gustavo A. R. Silva" <[email protected]> wrote on Thu, 7 May 2020 14:00:33 -0500:

Quoted 29 lines

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.

Quoted 8 lines

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by Gustavo A. R. Silva <[email protected]>

---

Acked-by Miquel Raynal <[email protected]>
maintainer Applied
#2 lore raw
In reply to Gustavo A. R. Silva
Quoted 11 lines · Gustavo A. R. Silva

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[];
};

[...]

Applied to https://git.kernel.org/pub/scm/linux/kernel/git/mtd/linux.git cfi/next, thanks!

Regards

Signature · 1 line
Vignesh