On Thu, 2026-09-10 at 20:22 +0800, Jiayuan Chen wrote:
> When an access goes past the struct and the last member is a flexible
> array, btf_struct_walk() folds the offset back into a single element with
> (off - moff) % t->size, but never checks that the element type has a size.
>
> BTF takes an empty struct, so this in program BTF
>
> /* event could be empty */
> struct event {
> #ifdef HAVE_TIMESTAMP
> __u64 ts;
> #endif
> };
>
> struct batch {
> int nr;
> struct event events[];
> };
>
> divides by zero at prog load time. Getting there needs a PTR_TO_BTF_ID that
> is not MEM_ALLOC, e.g. a plain read of a local kptr stashed in a map from a
> sleepable program.
>
> Oops: divide error: 0000 [#1] SMP KASAN PTI
> RIP: 0010:btf_struct_walk+0x53f/0x1570
> Call Trace:
> <TASK>
> btf_struct_access+0x42a/0xcd0
> check_ptr_to_btf_access+0x4dc/0x1160
> check_mem_access+0x3a45/0x8740
> check_load_mem+0x36a/0xd10
> do_check_common+0x3ef0/0xb210
> bpf_check+0x6d3b/0x8580
> bpf_prog_load+0xf7c/0x2720
> __sys_bpf+0xa83/0x3690
> __x64_sys_bpf+0xc7/0x150
> x64_sys_call+0x1f3f/0x27e0
> do_syscall_64+0xe5/0x610
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
> </TASK>
>
> Reject a zero-sized element type. The fixed array path in the same function
> already bails out on the same thing:
>
> btf_struct_walk()
> ...
> /* skip empty array */
> if (moff == mtrue_end)
> continue;
>
> msize /= total_nelems;
>
> Fixes: 9c5f8a1008a1 ("bpf: Support variable length array in tracing programs")
> Signed-off-by: Jiayuan Chen <[email protected]>
> ---
Acked-by: Eduard Zingerman <[email protected]>
