Re: [PATCH bpf-next] bpf: Fix kmemleak warnings for percpu hashmap

[Martin KaFai Lau <martin.lau@linux.dev>]

On 2/24/25 9:55 AM, Yonghong Song wrote:
> Vlad Poenaru from Meta reported the following kmemleak issues:
> 
>    ...
>    unreferenced object 0x606fd7c44ac8 (size 32):
>      comm "floodgate_agent", pid 5077, jiffies 4294746072
>      hex dump (first 32 bytes on cpu 32):
>        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>      backtrace (crc 0):
>        pcpu_alloc_noprof+0x730/0xeb0
>        bpf_map_alloc_percpu+0x69/0xc0
>        prealloc_init+0x9d/0x1b0
>        htab_map_alloc+0x363/0x510
>        map_create+0x215/0x3a0
>        __sys_bpf+0x16b/0x3e0
>        __x64_sys_bpf+0x18/0x20
>        do_syscall_64+0x7b/0x150
>        entry_SYSCALL_64_after_hwframe+0x4b/0x53
>    unreferenced object 0x606fd7c44ae8 (size 32):
>      comm "floodgate_agent", pid 5077, jiffies 4294746072
>      hex dump (first 32 bytes on cpu 32):
>        d3 08 00 00 00 00 00 00 d3 08 00 00 00 00 00 00  ................
>        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>      backtrace (crc d197b0fe):
>        pcpu_alloc_noprof+0x730/0xeb0
>        bpf_map_alloc_percpu+0x69/0xc0
>        prealloc_init+0x9d/0x1b0
>        htab_map_alloc+0x363/0x510
>        map_create+0x215/0x3a0
>        __sys_bpf+0x16b/0x3e0
>        __x64_sys_bpf+0x18/0x20
>        do_syscall_64+0x7b/0x150
>        entry_SYSCALL_64_after_hwframe+0x4b/0x53
>    ...
> 
> Further investigation shows the reason is due to not 8-byte aligned
> store of percpu pointer in htab_elem_set_ptr():
>    *(void __percpu **)(l->key + key_size) = pptr;
> 
> Note that the whole htab_elem alignment is 8 (for x86_64). If the key_size
> is 4, that means pptr is stored in a location which is 4 byte aligned but
> not 8 byte aligned. In mm/kmemleak.c, scan_block() scans the memory based
> on 8 byte stride, so it won't detect above pptr, hence reporting the memory
> leak.
> 
> In htab_map_alloc(), we already have
> 
>          htab->elem_size = sizeof(struct htab_elem) +
>                            round_up(htab->map.key_size, 8);
>          if (percpu)
>                  htab->elem_size += sizeof(void *);
>          else
>                  htab->elem_size += round_up(htab->map.value_size, 8);
> 
> So storing pptr with 8-byte alignment won't cause any problem and can fix
> kmemleak too.

Acked-by: Martin KaFai Lau <martin.lau@kernel.org>