bcachefs: Fix field spanning write warning

Struct with embedded VLA...

memcpy: detected field-spanning write (size 8) of single field "&gc->r.e" at fs/bcachefs/ec.c:465 (size 3)
WARNING: CPU: 1 PID: 936 at fs/bcachefs/ec.c:465 bch2_trigger_stripe+0x706/0x730
Modules linked in:
CPU: 1 UID: 0 PID: 936 Comm: mount.bcachefs Not tainted 6.14.0-rc6-ktest-00236-gefb0b5c62dbc ctrliq#55
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:bch2_trigger_stripe+0x706/0x730
Code: b4 00 01 b9 03 00 00 00 48 89 fb 48 c7 c7 33 54 da 81 48 89 d6 49 89 d6 48 c7 c2 c3 36 db 81 e8 60 54 c5 ff 48 89 df 4c 89 f2 <0f> 0b e9 5c fd ff ff e8 fe 5e 4e 00 bf 10 00 00 00 48 c7 c6 ff ff
RSP: 0018:ffff88817081f680 EFLAGS: 00010246
RAX: f8fe7dd1c56b5600 RBX: ffff888101265368 RCX: 0000000000000027
RDX: 0000000000000008 RSI: 00000000fffbffff RDI: ffff888101265368
RBP: 0000000000000000 R08: 000000000003ffff R09: ffff88817f1fe000
R10: 00000000000bfffd R11: 0000000000000004 R12: ffff8881012652c0
R13: 0000000000000000 R14: 0000000000000008 R15: ffff88817081f6c9
FS:  00007fc428bc7c80(0000) GS:ffff888179280000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffd3ee4a038 CR3: 000000010a9bc000 CR4: 0000000000750eb0
PKRU: 55555554
Call Trace:
 <TASK>
 ? __warn+0xce/0x1b0
 ? bch2_trigger_stripe+0x706/0x730
 ? report_bug+0x11b/0x1a0
 ? bch2_trigger_stripe+0x706/0x730
 ? handle_bug+0x5e/0x90
 ? exc_invalid_op+0x1a/0x50
 ? asm_exc_invalid_op+0x1a/0x20
 ? bch2_trigger_stripe+0x706/0x730
 bch2_gc_mark_key+0x2cf/0x430
 bch2_check_allocations+0x1a64/0x1ed0
 ? vsnprintf+0x1ad/0x420
 ? bch2_check_allocations+0x191f/0x1ed0
 bch2_run_recovery_passes+0x13b/0x2b0
 bch2_fs_recovery+0x9b7/0x1290
 ? __bch2_print+0xb2/0xf0
 ? bch2_printbuf_exit+0x1e/0x30
 ? print_mount_opts+0x153/0x180
 bch2_fs_start+0x274/0x3b0
 bch2_fs_get_tree+0x516/0x6e0
 vfs_get_tree+0x21/0xa0
 do_new_mount+0x153/0x350
 __x64_sys_mount+0x16c/0x1f0
 do_syscall_64+0x6c/0x140
 ? arch_exit_to_user_mode_prepare+0x9/0x40
 entry_SYSCALL_64_after_hwframe+0x4b/0x53

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: Kent Overstreet

fs/bcachefs/buckets.c

@@ -707,7 +707,7 @@ static int bch2_trigger_stripe_ptr(struct btree_trans *trans,
 		struct disk_accounting_pos acc;
 		memset(&acc, 0, sizeof(acc));
 		acc.type = BCH_DISK_ACCOUNTING_replicas;
-		memcpy(&acc.replicas, &m->r.e, replicas_entry_bytes(&m->r.e));
+		unsafe_memcpy(&acc.replicas, &m->r.e, replicas_entry_bytes(&m->r.e), "VLA");
 		gc_stripe_unlock(m);
 
 		acc.replicas.data_type = data_type;

fs/bcachefs/ec.c

@@ -462,7 +462,8 @@ int bch2_trigger_stripe(struct btree_trans *trans,
 				return ret;
 
 			if (gc)
-				memcpy(&gc->r.e, &acc.replicas, replicas_entry_bytes(&acc.replicas));
+				unsafe_memcpy(&gc->r.e, &acc.replicas,
+					      replicas_entry_bytes(&acc.replicas), "VLA");
 		}
 
 		if (old_s) {