btrfs: send: fix buffer overflow detection when copying path to cache entry

In the Linux kernel, the following vulnerability has been resolved:

btrfs: send: fix buffer overflow detection when copying path to cache entry

Starting with commit c0247d289e73 ("btrfs: send: annotate struct

name_cache_entry with __counted_by()") we annotated the variable length

array "name" from the name_cache_entry structure with __counted_by() to

improve overflow detection. However that alone was not correct, because

the length of that array does not match the "name_len" field - it matches

that plus 1 to include the NUL string terminator, so that makes a

fortified kernel think there's an overflow and report a splat like this:

strcpy: detected buffer overflow: 20 byte write of buffer size 19

WARNING: CPU: 3 PID: 3310 at __fortify_report+0x45/0x50

CPU: 3 UID: 0 PID: 3310 Comm: btrfs Not tainted 6.11.0-prnet #1

Hardware name: CompuLab Ltd. sbc-ihsw/Intense-PC2 (IPC2), BIOS IPC2_3.330.7 X64 03/15/2018

RIP: 0010:__fortify_report+0x45/0x50

Code: 48 8b 34 (...)

RSP: 0018:ffff97ebc0d6f650 EFLAGS: 00010246

RAX: 7749924ef60fa600 RBX: ffff8bf5446a521a RCX: 0000000000000027

RDX: 00000000ffffdfff RSI: ffff97ebc0d6f548 RDI: ffff8bf84e7a1cc8

RBP: ffff8bf548574080 R08: ffffffffa8c40e10 R09: 0000000000005ffd

R10: 0000000000000004 R11: ffffffffa8c70e10 R12: ffff8bf551eef400

R13: 0000000000000000 R14: 0000000000000013 R15: 00000000000003a8

FS: 00007fae144de8c0(0000) GS:ffff8bf84e780000(0000) knlGS:0000000000000000

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: 00007fae14691690 CR3: 00000001027a2003 CR4: 00000000001706f0

Call Trace:

<TASK>

? __warn+0x12a/0x1d0

? __fortify_report+0x45/0x50

? report_bug+0x154/0x1c0

? handle_bug+0x42/0x70

? exc_invalid_op+0x1a/0x50

? asm_exc_invalid_op+0x1a/0x20

? __fortify_report+0x45/0x50

__fortify_panic+0x9/0x10

__get_cur_name_and_parent+0x3bc/0x3c0

get_cur_path+0x207/0x3b0

send_extent_data+0x709/0x10d0

? find_parent_nodes+0x22df/0x25d0

? mas_nomem+0x13/0x90

? mtree_insert_range+0xa5/0x110

? btrfs_lru_cache_store+0x5f/0x1e0

? iterate_extent_inodes+0x52d/0x5a0

process_extent+0xa96/0x11a0

? __pfx_lookup_backref_cache+0x10/0x10

? __pfx_store_backref_cache+0x10/0x10

? __pfx_iterate_backrefs+0x10/0x10

? __pfx_check_extent_item+0x10/0x10

changed_cb+0x6fa/0x930

? tree_advance+0x362/0x390

? memcmp_extent_buffer+0xd7/0x160

send_subvol+0xf0a/0x1520

btrfs_ioctl_send+0x106b/0x11d0

? __pfx___clone_root_cmp_sort+0x10/0x10

_btrfs_ioctl_send+0x1ac/0x240

btrfs_ioctl+0x75b/0x850

__se_sys_ioctl+0xca/0x150

do_syscall_64+0x85/0x160

? __count_memcg_events+0x69/0x100

? handle_mm_fault+0x1327/0x15c0

? __se_sys_rt_sigprocmask+0xf1/0x180

? syscall_exit_to_user_mode+0x75/0xa0

? do_syscall_64+0x91/0x160

? do_user_addr_fault+0x21d/0x630

entry_SYSCALL_64_after_hwframe+0x76/0x7e

RIP: 0033:0x7fae145eeb4f

Code: 00 48 89 (...)

RSP: 002b:00007ffdf1cb09b0 EFLAGS: 00000246 ORIG_RAX: 0000000000000010

RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 00007fae145eeb4f

RDX: 00007ffdf1cb0ad0 RSI: 0000000040489426 RDI: 0000000000000004

RBP: 00000000000078fe R08: 00007fae144006c0 R09: 00007ffdf1cb0927

R10: 0000000000000008 R11: 0000000000000246 R12: 00007ffdf1cb1ce8

R13: 0000000000000003 R14: 000055c499fab2e0 R15: 0000000000000004

</TASK>

Fix this by not storing the NUL string terminator since we don't actually

need it for name cache entries, this way "name_len" corresponds to the

actual size of the "name" array. This requires marking the "name" array

field with __nonstring and using memcpy() instead of strcpy() as

recommended by the guidelines at:

https://github.com/KSPP/linux/issues/90