sfc: Replace zero-length array with flexible-array member
Subsystems (3)
PATCH 1/1 sfc: Replace zero-length array with flexible-array member
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]
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")
--- drivers/net/ethernet/sfc/falcon/net_driver.h | 2 +- drivers/net/ethernet/sfc/net_driver.h | 2 +- 2 files changed, 2 insertions(+), 2 deletions(-)
drivers/net/ethernet/sfc/falcon/net_driver.h +1 -1
@@ -288,7 +288,7 @@ struct ef4_rx_buffer {struct ef4_rx_page_state {dma_addr_t dma_addr; -unsigned int __pad[0 ]____cacheline_aligned ; +unsigned int __pad[]____cacheline_aligned ; };/**
drivers/net/ethernet/sfc/net_driver.h +1 -1
@@ -336,7 +336,7 @@ struct efx_rx_buffer {struct efx_rx_page_state {dma_addr_t dma_addr; -unsigned int __pad[0 ]____cacheline_aligned ; +unsigned int __pad[]____cacheline_aligned ; };/**
Signature · 1 line
2.25.0
Quoted 15 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[]; };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.
Quoted 6 lines
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]
These padding fields are only used to enforce alignment of the struct. But the patch is still ok.
Quoted 7 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] commit76497732932f("cxgb3/l2t: Fix undefined behaviour")Signed-off-by Gustavo A. R. Silva <[email protected]>
Quoted 32 lines
--- drivers/net/ethernet/sfc/falcon/net_driver.h | 2 +- drivers/net/ethernet/sfc/net_driver.h | 2 +- 2 files changed, 2 insertions(+), 2 deletions(-)drivers/net/ethernet/sfc/falcon/net_driver.h +1
-1@@ -288,7 +288,7 @@ struct ef4_rx_buffer {struct ef4_rx_page_state {dma_addr_t dma_addr; -unsigned int __pad[0 ]____cacheline_aligned ; +unsigned int __pad[]____cacheline_aligned ; };/** drivers/net/ethernet/sfc/net_driver.h +1
-1@@ -336,7 +336,7 @@ struct efx_rx_buffer {struct efx_rx_page_state {dma_addr_t dma_addr; -unsigned int __pad[0 ]____cacheline_aligned ; +unsigned int __pad[]____cacheline_aligned ; };/**
Quoted 19 lines · Martin Habets
Quoted 18 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[]; };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:
Quoted 7 lines
Quoted 3 lines
"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]
These padding fields are only used to enforce alignment of the struct. But the patch is still ok.
Correct.
Quoted 10 lines
Quoted 7 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] commit76497732932f("cxgb3/l2t: Fix undefined behaviour")Signed-off-by Gustavo A. R. Silva <[email protected]>Acked-by Martin Habets <[email protected]>
Thanks, Martin.
--
Gustavo
Quoted 32 lines
Quoted 32 lines
--- drivers/net/ethernet/sfc/falcon/net_driver.h | 2 +- drivers/net/ethernet/sfc/net_driver.h | 2 +- 2 files changed, 2 insertions(+), 2 deletions(-)drivers/net/ethernet/sfc/falcon/net_driver.h +1
-1@@ -288,7 +288,7 @@ struct ef4_rx_buffer {struct ef4_rx_page_state {dma_addr_t dma_addr; -unsigned int __pad[0 ]____cacheline_aligned ; +unsigned int __pad[]____cacheline_aligned ; };/** drivers/net/ethernet/sfc/net_driver.h +1
-1@@ -336,7 +336,7 @@ struct efx_rx_buffer {struct efx_rx_page_state {dma_addr_t dma_addr; -unsigned int __pad[0 ]____cacheline_aligned ; +unsigned int __pad[]____cacheline_aligned ; };/**
From: "Gustavo A. R. Silva" <[email protected]>
Date: Mon, 24 Feb 2020 18:06:47 -0600
Quoted 22 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]
Quoted 7 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] commit76497732932f("cxgb3/l2t: Fix undefined behaviour")Signed-off-by Gustavo A. R. Silva <[email protected]>
Applied.