Re: xdp/xsk.c: Possible bug in xdp_umem_reg version check

[Stanislav Fomichev <sdf@fomichev.me>]

On 07/09, Julian Schindel wrote:
> On 09.07.24 11:23, Magnus Karlsson wrote:
> > On Sun, 7 Jul 2024 at 17:06, Julian Schindel <mail@arctic-alpaca.de> wrote:
> >> Hi,
> >>
> >> [...]
> > Thank you for reporting this Julian. This seems to be a bug. If I
> > check the value of sizeof(struct xdp_umem_reg_v2), I get 32 bytes too
> > on my system, compiling with gcc 11.4. I am not a compiler guy so do
> > not know what the rules are for padding structs, but I read the
> > following from [0]:
> >
> > "Pad the entire struct to a multiple of 64-bits if the structure
> > contains 64-bit types - the structure size will otherwise differ on
> > 32-bit versus 64-bit. Having a different structure size hurts when
> > passing arrays of structures to the kernel, or if the kernel checks
> > the structure size, which e.g. the drm core does."
> >
> > I compiled for 64-bits and I believe you did too, but we still get
> > this padding. 
> Yes, I did also compile for 64-bits. If I understood the resource you
> linked correctly, the compiler automatically adding padding to align to
> 64-bit boundaries is expected for 64-bit platforms:
> 
> "[...] 32-bit platforms don’t necessarily align 64-bit values to 64-bit
> boundaries, but 64-bit platforms do. So we always need padding to the
> natural size to get this right."
> > What is sizeof(struct xdp_umem_reg) for you before the
> > patch that added tx_metadata_len?
> I would expect this to be the same as sizeof(struct xdp_umem_reg_v2)
> after the patch. I'm not sure how to check this with different kernel
> versions.
> 
> Maybe the following code helps show all the sizes
> of xdp_umem_reg[_v1/_v2] on my system (compiled with "gcc test.c -o
> test" using gcc 14.1.1):
> 
> #include <stdio.h>
> #include <sys/types.h>
> 
> typedef __uint32_t __u32;
> typedef __uint64_t __u64;
> 
> struct xdp_umem_reg_v1  {
>     __u64 addr; /* Start of packet data area */
>     __u64 len; /* Length of packet data area */
>     __u32 chunk_size;
>     __u32 headroom;
> };
> 
> struct xdp_umem_reg_v2 {
>     __u64 addr; /* Start of packet data area */
>     __u64 len; /* Length of packet data area */
>     __u32 chunk_size;
>     __u32 headroom;
>     __u32 flags;
> };
> 
> struct xdp_umem_reg {
>     __u64 addr; /* Start of packet data area */
>     __u64 len; /* Length of packet data area */
>     __u32 chunk_size;
>     __u32 headroom;
>     __u32 flags;
>     __u32 tx_metadata_len;
> };
> 
> int main() {
>     printf("__u32: \t\t\t %lu\n", sizeof(__u32));
>     printf("__u64: \t\t\t %lu\n", sizeof(__u64));
>     printf("xdp_umem_reg_v1: \t %lu\n", sizeof(struct xdp_umem_reg_v1));
>     printf("xdp_umem_reg_v2: \t %lu\n", sizeof(struct xdp_umem_reg_v2));
>     printf("xdp_umem_reg: \t\t %lu\n", sizeof(struct xdp_umem_reg));
> }
> 
> Running "./test" produced this output:
> 
> __u32:                   4
> __u64:                   8
> xdp_umem_reg_v1:         24
> xdp_umem_reg_v2:         32
> xdp_umem_reg:            32
> > [0]: https://www.kernel.org/doc/html/v5.4/ioctl/botching-up-ioctls.html

Hmm, true, this means our version check won't really work :-/ I don't
see a good way to solve it without breaking the uapi. We can either
add some new padding field to xdp_umem_reg to make it larger than _v2.
Or we can add a new flag to signify the presence of tx_metadata_len
and do the validation based on that.

Btw, what are you using to setup umem? Looking at libxsk, it does
`memset(&mr, 0, sizeof(mr));` which should clear the padding as well.