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Re: [PATCH][next] treewide: uapi: Replace zero-length arrays with flexib

To: Jason Gunthorpe <jgg@xxxxxxxx>
Subject: Re: [PATCH][next] treewide: uapi: Replace zero-length arrays with flexible-array members
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>, Kees Cook <keescook@xxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, x86@xxxxxxxxxx, dm-devel@xxxxxxxxxx, linux-m68k@xxxxxxxxxxxxxxxxxxxx, linux-mips@xxxxxxxxxxxxxxx, linux-s390@xxxxxxxxxxxxxxx, kvm@xxxxxxxxxxxxxxx, intel-gfx@xxxxxxxxxxxxxxxxxxxxx, dri-devel@xxxxxxxxxxxxxxxxxxxxx, netdev@xxxxxxxxxxxxxxx, bpf@xxxxxxxxxxxxxxx, linux-btrfs@xxxxxxxxxxxxxxx, linux-can@xxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux1394-devel@xxxxxxxxxxxxxxxxxxxxx, io-uring@xxxxxxxxxxxxxxx, lvs-devel@xxxxxxxxxxxxxxx, linux-mtd@xxxxxxxxxxxxxxxxxxx, kasan-dev@xxxxxxxxxxxxxxxx, linux-mmc@xxxxxxxxxxxxxxx, nvdimm@xxxxxxxxxxxxxxx, netfilter-devel@xxxxxxxxxxxxxxx, coreteam@xxxxxxxxxxxxx, linux-perf-users@xxxxxxxxxxxxxxx, linux-raid@xxxxxxxxxxxxxxx, linux-sctp@xxxxxxxxxxxxxxx, linux-stm32@xxxxxxxxxxxxxxxxxxxxxxxxxxxx, linux-arm-kernel@xxxxxxxxxxxxxxxxxxx, linux-scsi@xxxxxxxxxxxxxxx, target-devel@xxxxxxxxxxxxxxx, linux-usb@xxxxxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx, v9fs-developer@xxxxxxxxxxxxxxxxxxxxx, linux-rdma@xxxxxxxxxxxxxxx, alsa-devel@xxxxxxxxxxxxxxxx, linux-hardening@xxxxxxxxxxxxxxx
From: "Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx>
Date: Tue, 28 Jun 2022 02:58:25 +0200
On Mon, Jun 27, 2022 at 09:40:52PM -0300, Jason Gunthorpe wrote:
> On Mon, Jun 27, 2022 at 08:27:37PM +0200, Daniel Borkmann wrote:
> > On 6/27/22 8:04 PM, Gustavo A. R. Silva wrote:
> > > 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[2].
> > > 
> > > This code was transformed with the help of Coccinelle:
> > > (linux-5.19-rc2$ spatch --jobs $(getconf _NPROCESSORS_ONLN) --sp-file 
> > > script.cocci --include-headers --dir . > output.patch)
> > > 
> > > @@
> > > identifier S, member, array;
> > > type T1, T2;
> > > @@
> > > 
> > > struct S {
> > >    ...
> > >    T1 member;
> > >    T2 array[
> > > - 0
> > >    ];
> > > };
> > > 
> > > -fstrict-flex-arrays=3 is coming and we need to land these changes
> > > to prevent issues like these in the short future:
> > > 
> > > ../fs/minix/dir.c:337:3: warning: 'strcpy' will always overflow; 
> > > destination buffer has size 0,
> > > but the source string has length 2 (including NUL byte) [-Wfortify-source]
> > >           strcpy(de3->name, ".");
> > >           ^
> > > 
> > > Since these are all [0] to [] changes, the risk to UAPI is nearly zero. If
> > > this breaks anything, we can use a union with a new member name.
> > > 
> > > [1] https://en.wikipedia.org/wiki/Flexible_array_member
> > > [2] 
> > > https://www.kernel.org/doc/html/v5.16/process/deprecated.html#zero-length-and-one-element-arrays
> > > 
> > > Link: https://github.com/KSPP/linux/issues/78
> > > Build-tested-by: 
> > > https://lore.kernel.org/lkml/62b675ec.wKX6AOZ6cbE71vtF%25lkp@xxxxxxxxx/
> > > Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
> > > ---
> > > Hi all!
> > > 
> > > JFYI: I'm adding this to my -next tree. :)
> > 
> > Fyi, this breaks BPF CI:
> > 
> > https://github.com/kernel-patches/bpf/runs/7078719372?check_suite_focus=true
> > 
> >   [...]
> >   progs/map_ptr_kern.c:314:26: error: field 'trie_key' with variable sized 
> > type 'struct bpf_lpm_trie_key' not at the end of a struct or class is a GNU 
> > extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
> >           struct bpf_lpm_trie_key trie_key;
> >                                   ^
> 
> This will break the rdma-core userspace as well, with a similar
> error:
> 
> /usr/bin/clang-13 -DVERBS_DEBUG -Dibverbs_EXPORTS -Iinclude 
> -I/usr/include/libnl3 -I/usr/include/drm -g -O2 -fdebug-prefix-map=/__w/1/s=. 
> -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time 
> -D_FORTIFY_SOURCE=2 -Wmissing-prototypes -Wmissing-declarations 
> -Wwrite-strings -Wformat=2 -Wcast-function-type -Wformat-nonliteral 
> -Wdate-time -Wnested-externs -Wshadow -Wstrict-prototypes 
> -Wold-style-definition -Werror -Wredundant-decls -g -fPIC   -std=gnu11 -MD 
> -MT libibverbs/CMakeFiles/ibverbs.dir/cmd_flow.c.o -MF 
> libibverbs/CMakeFiles/ibverbs.dir/cmd_flow.c.o.d -o 
> libibverbs/CMakeFiles/ibverbs.dir/cmd_flow.c.o   -c ../libibverbs/cmd_flow.c
> In file included from ../libibverbs/cmd_flow.c:33:
> In file included from include/infiniband/cmd_write.h:36:
> In file included from include/infiniband/cmd_ioctl.h:41:
> In file included from include/infiniband/verbs.h:48:
> In file included from include/infiniband/verbs_api.h:66:
> In file included from include/infiniband/ib_user_ioctl_verbs.h:38:
> include/rdma/ib_user_verbs.h:436:34: error: field 'base' with variable sized 
> type 'struct ib_uverbs_create_cq_resp' not at the end of a struct or class is 
> a GNU extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
>         struct ib_uverbs_create_cq_resp base;
>                                         ^
> include/rdma/ib_user_verbs.h:644:34: error: field 'base' with variable sized 
> type 'struct ib_uverbs_create_qp_resp' not at the end of a struct or class is 
> a GNU extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
>         struct ib_uverbs_create_qp_resp base;
> 
> Which is why I gave up trying to change these..
> 
> Though maybe we could just switch off -Wgnu-variable-sized-type-not-at-end  
> during configuration ?

No. I think now we can easily workaround these sorts of problems with
something like this:

        struct flex {
                any_type any_member;
                union {
                        type array[0];
                        __DECLARE_FLEX_ARRAY(type, array_flex);
                };
        };

and use array_flex in kernel-space.

The same for the one-elment arrays in UAPI:

        struct flex {
                any_type any_member;
                union {
                        type array[1];
                        __DECLARE_FLEX_ARRAY(type, array_flex);
                };
        };

I'll use the idiom above to resolve all these warnings in a follow-up
patch. :)

Thanks
--
Gustavo

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