Re: [PATCH] arm64: asm-bug: Add .align 2 to the end of __BUG_ENTRY

[Jiangfeng Xiao]

On 2024/5/20 18:33, Mark Rutland wrote:

> I reckon we should spell out the reason for this a bit more; would you
> be happy with the commit message below?
> 
> | When CONFIG_DEBUG_BUGVERBOSE=n, we fail to add necessary padding bytes
> | to bug_table entries, and as a result the last entry in a bug table will
> | be ignored, potentially leading to an unexpected panic(). All prior
> | entries in the table will be handled correctly.
> | 
> | The arm64 ABI requires that struct fields of up to 8 bytes are
> | naturally-aligned, with padding added within a struct such that struct
> | are suitably aligned within arrays.
> | 
> | When CONFIG_DEBUG_BUGVERPOSE=y, the layout of a bug_entry is:
> | 
> | 	struct bug_entry {
> | 		signed int      bug_addr_disp;	// 4 bytes
> | 		signed int      file_disp;	// 4 bytes
> | 		unsigned short  line;		// 2 bytes
> | 		unsigned short  flags;		// 2 bytes
> | 	}
> | 
> | ... with 12 bytes total, requiring 4-byte alignment.
> | 
> | When CONFIG_DEBUG_BUGVERBOSE=n, the layout of a bug_entry is:
> | 
> | 	struct bug_entry {
> | 		signed int      bug_addr_disp;	// 4 bytes
> | 		unsigned short  flags;		// 2 bytes
> | 		< implicit padding >		// 2 bytes
> | 	}
> | 
> | ... with 8 bytes total, with 6 bytes of data and 2 bytes of trailing
> | padding, requiring 4-byte alginment.
> | 
> | When we create a bug_entry in assembly, we align the start of the entry
> | to 4 bytes, which implicitly handles padding for any prior entries.
> | However, we do not align the end of the entry, and so when
> | CONFIG_DEBUG_BUGVERBOSE=n, the final entry lacks the trailing padding
> | bytes.
> | 
> | For the main kernel image this is not a problem as find_bug() doesn't
> | depend on the trailing padding bytes when searching for entries:
> | 
> | 	for (bug = __start___bug_table; bug < __stop___bug_table; ++bug)
> | 		if (bugaddr == bug_addr(bug))
> | 			return bug;
> | 
> | However for modules, module_bug_finalize() depends on the trailing
> | bytes when calculating the number of entries:
> | 
> | 	mod->num_bugs = sechdrs[i].sh_size / sizeof(struct bug_entry);
> | 
> | ... and as the last bug_entry lacks the necessary padding bytes, this entry
> | will not be counted, e.g. in the case of a single entry:
> | 	
> | 	sechdrs[i].sh_size == 6
> | 	sizeof(struct bug_entry) == 8;
> | 
> | 	sechdrs[i].sh_size / sizeof(struct bug_entry) == 0;
> | 
> | Consequently module_find_bug() will miss the last bug_entry when it does:
> | 
> | 	for (i = 0; i < mod->num_bugs; ++i, ++bug)
> | 		if (bugaddr == bug_addr(bug))
> | 			goto out;	
> | 
> | ... which can lead to a kenrel panic due to an unhandled bug.
> | 
> | This can be demonstrated with the following module:
> | 
> | 	static int __init buginit(void)
> | 	{
> | 		WARN(1, "hello\n");
> | 		return 0;
> | 	}
> | 
> | 	static void __exit bugexit(void)
> | 	{
> | 	}
> | 
> | 	module_init(buginit);
> | 	module_exit(bugexit);
> | 	MODULE_LICENSE("GPL");
> | 
> | ... which will trigger a kernel panic when loaded:
> | 
> | 	------------[ cut here ]------------
> | 	hello
> | 	Unexpected kernel BRK exception at EL1
> | 	Internal error: BRK handler: 00000000f2000800 [#1] PREEMPT SMP
> | 	Modules linked in: hello(O+)
> | 	CPU: 0 PID: 50 Comm: insmod Tainted: G           O       6.9.1 #8
> | 	Hardware name: linux,dummy-virt (DT)
> | 	pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> | 	pc : buginit+0x18/0x1000 [hello]
> | 	lr : buginit+0x18/0x1000 [hello]
> | 	sp : ffff800080533ae0
> | 	x29: ffff800080533ae0 x28: 0000000000000000 x27: 0000000000000000
> | 	x26: ffffaba8c4e70510 x25: ffff800080533c30 x24: ffffaba8c4a28a58
> | 	x23: 0000000000000000 x22: 0000000000000000 x21: ffff3947c0eab3c0
> | 	x20: ffffaba8c4e3f000 x19: ffffaba846464000 x18: 0000000000000006
> | 	x17: 0000000000000000 x16: ffffaba8c2492834 x15: 0720072007200720
> | 	x14: 0720072007200720 x13: ffffaba8c49b27c8 x12: 0000000000000312
> | 	x11: 0000000000000106 x10: ffffaba8c4a0a7c8 x9 : ffffaba8c49b27c8
> | 	x8 : 00000000ffffefff x7 : ffffaba8c4a0a7c8 x6 : 80000000fffff000
> | 	x5 : 0000000000000107 x4 : 0000000000000000 x3 : 0000000000000000
> | 	x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff3947c0eab3c0
> | 	Call trace:
> | 	 buginit+0x18/0x1000 [hello]
> | 	 do_one_initcall+0x80/0x1c8
> | 	 do_init_module+0x60/0x218
> | 	 load_module+0x1ba4/0x1d70
> | 	 __do_sys_init_module+0x198/0x1d0
> | 	 __arm64_sys_init_module+0x1c/0x28
> | 	 invoke_syscall+0x48/0x114
> | 	 el0_svc_common.constprop.0+0x40/0xe0
> | 	 do_el0_svc+0x1c/0x28
> | 	 el0_svc+0x34/0xd8
> | 	 el0t_64_sync_handler+0x120/0x12c
> | 	 el0t_64_sync+0x190/0x194
> | 	Code: d0ffffe0 910003fd 91000000 9400000b (d4210000)
> | 	---[ end trace 0000000000000000 ]---
> | 	Kernel panic - not syncing: BRK handler: Fatal exception
> | 
> | Fix this by always aligning the end of a bug_entry to 4 bytes, which is
> | correct regardless of CONFIG_DEBUG_BUGVERBOSE.
> | 
> | Fixes: 9fb7410f955f ("arm64/BUG: Use BRK instruction for generic BUG traps")
> | Signed-off-by: Yuanbin Xie <xieyuanbin1@xxxxxxxxxx>
> | Signed-off-by: Jiangfeng Xiao <xiaojiangfeng@xxxxxxxxxx>
> 
> With that:
> 
> Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>
> 
> Mark.
> 

Thank you very much for your patience and professional review.

Indeed, we have not clearly expressed the specific layout
of `struct bug_entry` under the arm64 ABI
and how it causes the problem. We have only briefly described
that the assembly code does not consistent with
the binary layout of the C struct.

Your commit messge is much more accurate
in describing the problem, and we couldn't agree more.

I will follow your advice to submit our V2 patch.