Re: [Xen-devel] [PATCH v6 04/12] xen: add basic hypervisor filesystem support

[Jan Beulich <jbeulich@suse.com>]

On 04.03.2020 16:14, Jürgen Groß wrote:
> On 04.03.20 16:07, Jan Beulich wrote:
>> On 04.03.2020 15:39, Jürgen Groß wrote:
>>> On 04.03.20 14:03, Jan Beulich wrote:
>>>> On 04.03.2020 13:00, Jürgen Groß wrote:
>>>>> On 03.03.20 17:59, Jan Beulich wrote:
>>>>>> On 26.02.2020 13:46, Juergen Gross wrote:
>>>>>>> --- /dev/null
>>>>>>> +++ b/xen/common/hypfs.c
>>>>>>> @@ -0,0 +1,349 @@
>>>>>>> +/******************************************************************************
>>>>>>> + *
>>>>>>> + * hypfs.c
>>>>>>> + *
>>>>>>> + * Simple sysfs-like file system for the hypervisor.
>>>>>>> + */
>>>>>>> +
>>>>>>> +#include <xen/err.h>
>>>>>>> +#include <xen/guest_access.h>
>>>>>>> +#include <xen/hypercall.h>
>>>>>>> +#include <xen/hypfs.h>
>>>>>>> +#include <xen/lib.h>
>>>>>>> +#include <xen/rwlock.h>
>>>>>>> +#include <public/hypfs.h>
>>>>>>> +
>>>>>>> +#ifdef CONFIG_COMPAT
>>>>>>> +#include <compat/hypfs.h>
>>>>>>> +CHECK_hypfs_direntry;
>>>>>>> +#undef CHECK_hypfs_direntry
>>>>>>> +#define CHECK_hypfs_direntry struct xen_hypfs_direntry
>>>>>>
>>>>>> I'm struggling to see why you need this #undef and #define.
>>>>>
>>>>> Without those I get:
>>>>>
>>>>> In file included from /home/gross/xen/unstable/xen/include/compat/xen.h:3:0,
>>>>>                     from /home/gross/xen/unstable/xen/include/xen/shared.h:6,
>>>>>                     from /home/gross/xen/unstable/xen/include/xen/sched.h:8,
>>>>>                     from /home/gross/xen/unstable/xen/include/asm/paging.h:29,
>>>>>                     from
>>>>> /home/gross/xen/unstable/xen/include/asm/guest_access.h:1,
>>>>>                     from
>>>>> /home/gross/xen/unstable/xen/include/xen/guest_access.h:1,
>>>>>                     from hypfs.c:9:
>>>>> /home/gross/xen/unstable/xen/include/xen/compat.h:134:32: error:
>>>>> redefinition of ‘__checkFstruct_hypfs_direntry__flags’
>>>>>     #define CHECK_NAME_(k, n, tag) __check ## tag ## k ## _ ## n
>>>>>                                    ^
>>>>> /home/gross/xen/unstable/xen/include/xen/compat.h:166:34: note: in
>>>>> definition of macro ‘CHECK_FIELD_COMMON_’
>>>>>     static inline int __maybe_unused name(k xen_ ## n *x, k compat_ ## n *c) \
>>>>>                                      ^~~~
>>>>> /home/gross/xen/unstable/xen/include/xen/compat.h:176:28: note: in
>>>>> expansion of macro ‘CHECK_NAME_’
>>>>>         CHECK_FIELD_COMMON_(k, CHECK_NAME_(k, n ## __ ## f, F), n, f)
>>>>>                                ^~~~~~~~~~~
>>>>> /home/gross/xen/unstable/xen/include/compat/xlat.h:775:5: note: in
>>>>> expansion of macro ‘CHECK_FIELD_’
>>>>>         CHECK_FIELD_(struct, hypfs_direntry, flags); \
>>>>>         ^~~~~~~~~~~~
>>>>> /home/gross/xen/unstable/xen/include/compat/xlat.h:782:5: note: in
>>>>> expansion of macro ‘CHECK_hypfs_direntry’
>>>>>         CHECK_hypfs_direntry; \
>>>>>         ^~~~~~~~~~~~~~~~~~~~
>>>>> hypfs.c:19:1: note: in expansion of macro ‘CHECK_hypfs_dirlistentry’
>>>>>     CHECK_hypfs_dirlistentry;
>>>>>     ^~~~~~~~~~~~~~~~~~~~~~~~
>>>>> /home/gross/xen/unstable/xen/include/xen/compat.h:134:32: note: previous
>>>>> definition of ‘__checkFstruct_hypfs_direntry__flags’ was here
>>>>>     #define CHECK_NAME_(k, n, tag) __check ## tag ## k ## _ ## n
>>>>>                                    ^
>>>>> /home/gross/xen/unstable/xen/include/xen/compat.h:166:34: note: in
>>>>> definition of macro ‘CHECK_FIELD_COMMON_’
>>>>>     static inline int __maybe_unused name(k xen_ ## n *x, k compat_ ## n *c) \
>>>>>                                      ^~~~
>>>>> /home/gross/xen/unstable/xen/include/xen/compat.h:176:28: note: in
>>>>> expansion of macro ‘CHECK_NAME_’
>>>>>         CHECK_FIELD_COMMON_(k, CHECK_NAME_(k, n ## __ ## f, F), n, f)
>>>>>                                ^~~~~~~~~~~
>>>>> /home/gross/xen/unstable/xen/include/compat/xlat.h:775:5: note: in
>>>>> expansion of macro ‘CHECK_FIELD_’
>>>>>         CHECK_FIELD_(struct, hypfs_direntry, flags); \
>>>>>         ^~~~~~~~~~~~
>>>>> hypfs.c:18:1: note: in expansion of macro ‘CHECK_hypfs_direntry’
>>>>>     CHECK_hypfs_direntry;
>>>>
>>>> Which suggests to me that the explicit CHECK_hypfs_direntry invocation
>>>> is unneeded, as it's getting verified as part of the invocation of
>>>> CHECK_hypfs_dirlistentry.
>>>
>>> Ah, right. This is working. Will change.
>>>
>>>>
>>>>>>> +int hypfs_write_leaf(struct hypfs_entry_leaf *leaf,
>>>>>>> +                     XEN_GUEST_HANDLE_PARAM(void) uaddr, unsigned long ulen)
>>>>>>> +{
>>>>>>> +    char *buf;
>>>>>>> +    int ret;
>>>>>>> +
>>>>>>> +    if ( ulen > leaf->e.size )
>>>>>>> +        return -ENOSPC;
>>>>>>> +
>>>>>>> +    if ( leaf->e.type != XEN_HYPFS_TYPE_STRING &&
>>>>>>> +         leaf->e.type != XEN_HYPFS_TYPE_BLOB && ulen != leaf->e.size )
>>>>>>> +        return -EDOM;
>>>>>>
>>>>>> Why the exception of string and blob? My concern about the
>>>>>> meaning of a partially written entry (without its size having
>>>>>> changed) remains.
>>>>>
>>>>> It is perfectly valid to write a shorter string into a character
>>>>> array. I could drop the blob here, but in the end I think allowing
>>>>> for a blob to change the size should be fine.
>>>>
>>>> But shouldn't this then also adjust the recorded size?
>>>
>>> No, this is the max size of the buffer (you can have a look at patch 9
>>> where the size is set to the provided space for custom and string
>>> parameters).
>>
>> If I'm not mistaken it is hypfs_read_leaf() which processes read
>> requests for strings. Yet that copies entry->size bytes, not the
>> potentially smaller strlen()-bounded payload. Things would be
> 
> There is no risk of leaking problematic data here.

I didn't think of leaks, but rather of consumers looking at the
size and strlen() and getting confused about the mismatch.

Jan

>> even worse for BLOB-type entries, where one couldn't even look
>> for a nul terminator to determine actual payload size.
> 
> Right, this would probably require a blob-specific read function, in
> case the blob is of variable length.
> 
> 
> Juergen
> 


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