Messages in this thread | | | Subject | Re: [PATCH v2] tty: n_hdlc: Use flexible-array member and struct_size() helper | From | Jiri Slaby <> | Date | Thu, 23 Jan 2020 07:16:29 +0100 |
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On 21. 01. 20, 18:21, Gustavo A. R. Silva wrote: > Old code in the kernel uses 1-byte and 0-byte arrays to indicate the > presence of a "variable length array": > > struct something { > int length; > u8 data[1]; > }; > > struct something *instance; > > instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL); > instance->length = size; > memcpy(instance->data, source, size); > > There is also 0-byte arrays. Both cases pose confusion for things like > sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism > to declare variable-length types such as the one above is a flexible array > member[2] which need to be the last member of a structure and empty-sized: > > struct something { > int stuff; > u8 data[]; > }; > > Also, 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 > inadvertenly introduced[3] to the codebase from now on. > > Lastly, make use of the struct_size() helper to safely calculate the > allocation size for instances of struct n_hdlc_buf and avoid any potential > type mistakes[4][5]. > > [1] https://github.com/KSPP/linux/issues/21 > [2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html > [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour") > [4] https://lore.kernel.org/lkml/60e14fb7-8596-e21c-f4be-546ce39e7bdb@embeddedor.com/ > [5] commit 553d66cb1e86 ("iommu/vt-d: Use struct_size() helper") > > Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Now it looks good to me: Reviewed-by: Jiri Slaby <jslaby@suse.cz>
thanks, -- js suse labs
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