Re: [PATCH v8 8/9] tmpfs: Expose filesystem features via sysfs

["André Almeida" <andrealmeid@igalia.com>]

Hi Nathan,

Em 31/10/2024 02:18, Nathan Chancellor escreveu:
> Hi André,
> 
> On Mon, Oct 21, 2024 at 01:37:24PM -0300, André Almeida wrote:
>> Expose filesystem features through sysfs, so userspace can query if
>> tmpfs support casefold.
>>
>> This follows the same setup as defined by ext4 and f2fs to expose
>> casefold support to userspace.
>>
>> Signed-off-by: André Almeida <andrealmeid@igalia.com>
>> Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
>> ---
>>   mm/shmem.c | 37 +++++++++++++++++++++++++++++++++++++
>>   1 file changed, 37 insertions(+)
>>
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index ea01628e443423d82d44277e085b867ab9bf4b28..0739143d1419c732359d3a3c3457c3acb90c5b22 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -5546,3 +5546,40 @@ struct page *shmem_read_mapping_page_gfp(struct address_space *mapping,
>>   	return page;
>>   }
>>   EXPORT_SYMBOL_GPL(shmem_read_mapping_page_gfp);
>> +
>> +#if defined(CONFIG_SYSFS) && defined(CONFIG_TMPFS)
>> +#if IS_ENABLED(CONFIG_UNICODE)
>> +static DEVICE_STRING_ATTR_RO(casefold, 0444, "supported");
>> +#endif
>> +
>> +static struct attribute *tmpfs_attributes[] = {
>> +#if IS_ENABLED(CONFIG_UNICODE)
>> +	&dev_attr_casefold.attr.attr,
>> +#endif
>> +	NULL
>> +};
>> +
>> +static const struct attribute_group tmpfs_attribute_group = {
>> +	.attrs = tmpfs_attributes,
>> +	.name = "features"
>> +};
>> +
>> +static struct kobject *tmpfs_kobj;
>> +
>> +static int __init tmpfs_sysfs_init(void)
>> +{
>> +	int ret;
>> +
>> +	tmpfs_kobj = kobject_create_and_add("tmpfs", fs_kobj);
>> +	if (!tmpfs_kobj)
>> +		return -ENOMEM;
>> +
>> +	ret = sysfs_create_group(tmpfs_kobj, &tmpfs_attribute_group);
>> +	if (ret)
>> +		kobject_put(tmpfs_kobj);
>> +
>> +	return ret;
>> +}
>> +
>> +fs_initcall(tmpfs_sysfs_init);
>> +#endif /* CONFIG_SYSFS && CONFIG_TMPFS */
>>
>> -- 
>> 2.47.0
>>
> 
> This change as commit 5132f08bd332 ("tmpfs: Expose filesystem features
> via sysfs") in -next introduces a kCFI violation when accessing
> /sys/fs/tmpfs/features/casefold. An attribute group created with
> sysfs_create_group() has ->sysfs_ops() set to kobj_sysfs_ops, which has
> a ->show() value of kobj_attr_show(). When kobj_attr_show() goes to call
> the attribute's ->show() value after container_of(), there will be a
> type mismatch in the case of the casefold attr, as it was defined with a
> ->show() value of device_show_string() but that does not match the type
> of ->show() in 'struct kobj_attribute'.
> 
> I can easily reproduce this with the following commands:
> 
>    $ printf 'CONFIG_%s=y\n' CFI_CLANG UNICODE >kernel/configs/repro.config
> 
>    $ make -skj"$(nproc)" ARCH=arm64 LLVM=1 mrproper virtconfig repro.config Image.gz
>    ...
> 
>    $ curl -LSs https://github.com/ClangBuiltLinux/boot-utils/releases/download/20230707-182910/arm64-rootfs.cpio.zst | zstd -d >rootfs.cpio
> 
>    $ qemu-system-aarch64 \
>        -display none \
>        -nodefaults \
>        -cpu max,pauth-impdef=true \
>        -machine virt,gic-version=max,virtualization=true \
>        -append 'console=ttyAMA0 earlycon rdinit=/bin/sh' \
>        -kernel arch/arm64/boot/Image.gz \
>        -initrd rootfs.cpio \
>        -m 512m \
>        -serial mon:stdio
>    ...
>    # mount -t sysfs sys /sys
>    # cat /sys/fs/tmpfs/features/casefold
>    [   70.558496] CFI failure at kobj_attr_show+0x2c/0x4c (target: device_show_string+0x0/0x38; expected type: 0xc527b809)
>    [   70.560018] Internal error: Oops - CFI: 00000000f2008228 [#1] PREEMPT SMP
>    [   70.560647] Modules linked in:
>    [   70.561770] CPU: 0 UID: 0 PID: 46 Comm: cat Not tainted 6.12.0-rc4-00008-g5132f08bd332 #1
>    [   70.562429] Hardware name: linux,dummy-virt (DT)
>    [   70.562897] pstate: 21402009 (nzCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
>    [   70.563377] pc : kobj_attr_show+0x2c/0x4c
>    [   70.563674] lr : sysfs_kf_seq_show+0xb4/0x130
>    [   70.563987] sp : ffff80008043bac0
>    [   70.564236] x29: ffff80008043bac0 x28: 000000007ffff001 x27: 0000000000000000
>    [   70.564877] x26: 0000000001000000 x25: 000000007ffff001 x24: 0000000000000001
>    [   70.565339] x23: fff000000238a000 x22: ffff9fa31a3996f8 x21: fff00000023fc000
>    [   70.565806] x20: fff000000201df80 x19: fff000000238b000 x18: 0000000000000000
>    [   70.566273] x17: 00000000c527b809 x16: 00000000df43c25c x15: fff000001fef8200
>    [   70.566727] x14: 0000000000000000 x13: fff00000022450f0 x12: 0000000000001000
>    [   70.567177] x11: fff00000023fc000 x10: 0000000000000000 x9 : ffff9fa31a18fac4
>    [   70.567682] x8 : ffff9fa319badde4 x7 : 0000000000000000 x6 : 000000000000003f
>    [   70.568138] x5 : 0000000000000040 x4 : 0000000000000000 x3 : 0000000000000004
>    [   70.568585] x2 : fff00000023fc000 x1 : ffff9fa31a881f90 x0 : fff000000201df80
>    [   70.569169] Call trace:
>    [   70.569389]  kobj_attr_show+0x2c/0x4c
>    [   70.569706]  sysfs_kf_seq_show+0xb4/0x130
>    [   70.570020]  kernfs_seq_show+0x44/0x54
>    [   70.570280]  seq_read_iter+0x14c/0x4b0
>    [   70.570543]  kernfs_fop_read_iter+0x60/0x198
>    [   70.570820]  copy_splice_read+0x1f0/0x2f4
>    [   70.571092]  splice_direct_to_actor+0xf4/0x2e0
>    [   70.571376]  do_splice_direct+0x68/0xb8
>    [   70.571626]  do_sendfile+0x1e8/0x488
>    [   70.571874]  __arm64_sys_sendfile64+0xe0/0x12c
>    [   70.572161]  invoke_syscall+0x58/0x114
>    [   70.572424]  el0_svc_common+0xa8/0xdc
>    [   70.572676]  do_el0_svc+0x1c/0x28
>    [   70.572910]  el0_svc+0x38/0x68
>    [   70.573132]  el0t_64_sync_handler+0x90/0xfc
>    [   70.573394]  el0t_64_sync+0x190/0x19
>    [   70.574001] Code: 72970131 72b8a4f1 6b11021f 54000040 (d4304500)
>    [   70.574635] ---[ end trace 0000000000000000 ]---
> 
> I am not sure if there is a better API exists or if a local copy should
> be rolled but I think the current scheme is definitely wrong because
> there is no 'struct device' here.
> 

Thank you for the report, I'm trying to fix it, it seems I have used 
something for device drivers in a filesystem. I wonder how btrfs doesn't 
get this error, since tmpfs_sysfs_init() is very similar to 
btrfs_init_sysfs().

> If there is any patch I can test or further information I can provide, I
> am more than happy to do so.
> 
> Cheers,
> Nathan