Messages in this thread | | | Date | Mon, 15 Jun 2020 15:39:17 -0500 | From | "Gustavo A. R. Silva" <> | Subject | [GIT PULL] flexible-array member conversion patches for 5.8-rc2 |
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The following changes since commit b3a9e3b9622ae10064826dccb4f7a52bd88c7407:
Linux 5.8-rc1 (2020-06-14 12:45:04 -0700)
are available in the Git repository at:
git://git.kernel.org/pub/scm/linux/kernel/git/gustavoars/linux.git tags/flex-array-conversions-5.8-rc2
for you to fetch changes up to 4caeb3acd21ffa2496fc3e94398019d4b3415523:
w1: Replace zero-length array with flexible-array (2020-06-15 15:20:19 -0500)
---------------------------------------------------------------- flexible-array member conversion patches for 5.8-rc2
Hi Linus,
Please, pull the following patches that replace zero-length arrays with flexible-array members.
Notice that all of these patches have been baking in linux-next for two development cycles now.
There is a regular need in the kernel to provide a way to declare having a dynamically sized set of trailing elements in a structure. Kernel code should always use “flexible array members”[1] for these cases. The older style of one-element or zero-length arrays should no longer be used.
In older C code, dynamically sized trailing elements were done by specifying a one-element array at the end of a structure:
struct something { size_t count; struct foo items[1]; };
This led to fragile size calculations via sizeof() (which would need to remove the size of the single trailing element to get a correct size of the “header”). A GNU C extension[2] was introduced to allow for zero-length arrays, to avoid these kinds of size problems:
struct something { size_t count; struct foo items[0]; };
But this led to other problems, and didn’t solve some problems shared by both styles, like not being able to detect when such an array is accidentally being used _not_ at the end of a structure (which could happen directly, or when such a struct was in unions, structs of structs, etc).
C99 introduced “flexible array members”, which lacks a numeric size for the array declaration entirely:
struct something { size_t count; struct foo items[]; };
This is the way the kernel expects dynamically sized trailing elements to be declared. It allows the compiler to generate errors when the flexible array does not occur last in the structure, which helps to prevent some kind of undefined behavior[3] bugs from being inadvertently introduced to the codebase. It also allows the compiler to correctly analyze array sizes (via sizeof(), CONFIG_FORTIFY_SOURCE, and CONFIG_UBSAN_BOUNDS). For instance, there is no mechanism that warns us that the following application of the sizeof() operator to a zero-length array always results in zero:
struct something { size_t count; struct foo items[0]; };
struct something *instance;
instance = kmalloc(struct_size(instance, items, count), GFP_KERNEL); instance->count = count;
size = sizeof(instance->items) * instance->count; memcpy(instance->items, source, size);
At the last line of code above, size turns out to be zero, when one might have thought it represents the total size in bytes of the dynamic memory recently allocated for the trailing array items. Here are a couple examples of this issue[4][5]. Instead, flexible array members have incomplete type, and so the sizeof() operator may not be applied[2], so any misuse of such operators will be immediately noticed at build time.
With respect to one-element arrays, one has to be acutely aware that such arrays occupy at least as much space as a single object of the type[2], hence they contribute to the size of the enclosing structure. This is prone to error every time people want to calculate the total size of dynamic memory to allocate for a structure containing an array of this kind as a member:
struct something { size_t count; struct foo items[1]; };
struct something *instance;
instance = kmalloc(struct_size(instance, items, count - 1), GFP_KERNEL); instance->count = count;
size = sizeof(instance->items) * instance->count; memcpy(instance->items, source, size);
In the example above, we had to remember to calculate count - 1 when using the struct_size() helper, otherwise we would have –unintentionally– allocated memory for one too many items objects. The cleanest and least error-prone way to implement this is through the use of a flexible array member:
struct something { size_t count; struct foo items[]; };
struct something *instance;
instance = kmalloc(struct_size(instance, items, count), GFP_KERNEL); instance->count = count;
size = sizeof(instance->items[0]) * instance->count; memcpy(instance->items, source, size);
Thanks -- Gustavo
[1] https://en.wikipedia.org/wiki/Flexible_array_member [2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html [3] https://git.kernel.org/linus/76497732932f15e7323dc805e8ea8dc11bb587cf [4] https://git.kernel.org/linus/f2cd32a443da694ac4e28fbf4ac6f9d5cc63a539 [5] https://git.kernel.org/linus/ab91c2a89f86be2898cee208d492816ec238b2cf
---------------------------------------------------------------- Gustavo A. R. Silva (41): firmware: dmi-sysfs: Replace zero-length array with flexible-array member firmware: google: memconsole: Replace zero-length array with flexible-array member firmware: google: vpd: Replace zero-length array with flexible-array member aio: Replace zero-length array with flexible-array ARM: tegra: Replace zero-length array with flexible-array dmaengine: Replace zero-length array with flexible-array can: peak_canfd: Replace zero-length array with flexible-array can: Replace zero-length array with flexible-array crypto: Replace zero-length array with flexible-array drbd: Replace zero-length array with flexible-array drm/edid: Replace zero-length array with flexible-array cb710: Replace zero-length array with flexible-array firewire: ohci: Replace zero-length array with flexible-array FS-Cache: Replace zero-length array with flexible-array ia64: kernel: unwind_i.h: Replace zero-length array with flexible-array samples: mei: Replace zero-length array with flexible-array ibft: Replace zero-length array with flexible-array jffs2: Replace zero-length array with flexible-array KVM: Replace zero-length array with flexible-array kexec: Replace zero-length array with flexible-array keys: encrypted-type: Replace zero-length array with flexible-array kprobes: Replace zero-length array with flexible-array libata: Replace zero-length array with flexible-array tools/testing/nvdimm: Replace zero-length array with flexible-array block: Replace zero-length array with flexible-array oprofile: Replace zero-length array with flexible-array firmware: pcdp: Replace zero-length array with flexible-array media: pwc: Replace zero-length array with flexible-array rapidio: Replace zero-length array with flexible-array RxRPC: Replace zero-length array with flexible-array phy: samsung: Replace zero-length array with flexible-array sctp: Replace zero-length array with flexible-array ima: Replace zero-length array with flexible-array ASoC: SOF: Replace zero-length array with flexible-array Squashfs: Replace zero-length array with flexible-array stm class: Replace zero-length array with flexible-array dmaengine: tegra-apb: Replace zero-length array with flexible-array tifm: Replace zero-length array with flexible-array soc: ti: Replace zero-length array with flexible-array tracing/probe: Replace zero-length array with flexible-array w1: Replace zero-length array with flexible-array
arch/ia64/kernel/unwind_i.h | 2 +- block/partitions/ldm.h | 2 +- drivers/amba/tegra-ahb.c | 2 +- drivers/block/drbd/drbd_int.h | 2 +- drivers/block/drbd/drbd_protocol.h | 8 +++--- drivers/crypto/chelsio/chcr_crypto.h | 8 +++--- drivers/dma/milbeaut-hdmac.c | 2 +- drivers/dma/milbeaut-xdmac.c | 2 +- drivers/dma/moxart-dma.c | 2 +- drivers/dma/tegra20-apb-dma.c | 2 +- drivers/dma/ti/edma.c | 2 +- drivers/dma/ti/k3-udma.c | 2 +- drivers/dma/timb_dma.c | 2 +- drivers/firewire/core-cdev.c | 2 +- drivers/firewire/core-transaction.c | 2 +- drivers/firewire/core.h | 2 +- drivers/firewire/nosy.c | 2 +- drivers/firewire/ohci.c | 2 +- drivers/firmware/dmi-sysfs.c | 2 +- drivers/firmware/google/memconsole-coreboot.c | 2 +- drivers/firmware/google/vpd.c | 2 +- drivers/firmware/iscsi_ibft.c | 2 +- drivers/firmware/pcdp.h | 2 +- drivers/hwtracing/stm/policy.c | 2 +- drivers/hwtracing/stm/stm.h | 4 +-- drivers/media/usb/pwc/pwc.h | 2 +- drivers/net/can/peak_canfd/peak_pciefd_main.c | 4 +-- drivers/oprofile/cpu_buffer.h | 2 +- drivers/phy/samsung/phy-samsung-usb2.h | 2 +- drivers/rapidio/rio-scan.c | 2 +- drivers/soc/ti/knav_qmss.h | 2 +- drivers/w1/w1_netlink.h | 4 +-- fs/aio.c | 2 +- fs/jffs2/nodelist.h | 2 +- fs/jffs2/summary.h | 4 +-- fs/squashfs/squashfs_fs.h | 16 ++++++------ include/drm/drm_displayid.h | 2 +- include/keys/encrypted-type.h | 2 +- include/keys/rxrpc-type.h | 4 +-- include/linux/can/skb.h | 2 +- include/linux/cb710.h | 2 +- include/linux/dmaengine.h | 4 +-- include/linux/fscache-cache.h | 2 +- include/linux/kexec.h | 2 +- include/linux/kprobes.h | 2 +- include/linux/kvm_host.h | 2 +- include/linux/libata.h | 2 +- include/linux/sctp.h | 36 +++++++++++++-------------- include/linux/tifm.h | 2 +- kernel/trace/trace_probe.h | 2 +- samples/mei/mei-amt-version.c | 2 +- security/integrity/integrity.h | 4 +-- sound/soc/sof/probe.h | 8 +++--- tools/testing/nvdimm/test/nfit_test.h | 6 ++--- 54 files changed, 96 insertions(+), 96 deletions(-)
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