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SubjectRe: [PATCH 0/3] Extended file stat functions [ver #2]
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On 2010-06-30, at 17:15, David Howells wrote:
> Andreas Dilger <adilger@dilger.ca> wrote:
>>> Passing -1 (or ULONGLONG_MAX) to get everything would be reasonable.
>>
>> NOOOO. That is exactly what we _don't_ want, since it makes it impossible
>> for the kernel to actually understand which fields the application is ready
>> to handle. If the application always uses XSTAT_QUERY_ALL, instead of "-1",
>> then the kernel can easily tell which fields are present in the userspace
>> structure, and what it should avoid touching.
>>
>> If applications start using "-1" to mean "all fields", then it will work so
>> long as the kernel and userspace agree on the size of struct xstat, but as
>> soon as the kernel understands some new field, but userspace does not, the
>> application will segfault or clobber random memory because the kernel thinks
>> it is asking for XSTAT_QUERY_NEXT_NEW_FIELD|... when it really isn't asking
>> for that at all.
>
> As long as the field bits allocated in order and the extra results are tacked
> on in bit number order, will it actually be a problem? Userspace must know how to deal with all the bits up to the last one it knows about; anything beyond that is irrelevant.

The patch you sent seems to get this right, but just for completeness, I'll answer in this thread. Using the new struct as an example:

#define XSTAT_REQUEST_GEN 0x00001000ULL
#define XSTAT_REQUEST_DATA_VERSION 0x00002000ULL
struct xstat {
:
:
unsigned long long st_data_version;
unsigned long long st_result_mask;
unsigned long long st_extra_results[0];
}
An app "today" would allocate a struct xstat that ends at st_result_mask, and "today's" kernel will not know anything about flags beyond *_DATA_VERSION. Even if today's app incorrectly uses request_mask = ~0ULL nothing will break until the kernel code changes.

If a future kernel gets a new static field at st_extra_results (say unsigned long long st_ino_high) with a new flag XSTAT_REQUEST_INO_HIGH 0x000040000ULL the kernel will think that the old app is requesting this field, and will fill in the 64-bit field at st_extra_results[1] (which the old app didn't allocate space for, nor does it understand) and may get a segfault, or stack smashing, or random heap corruption.
> What would you have me do? Return an error if a request is made that the
> kernel doesn't support? That's bad too. This can be handled simply by
> clearing the result bit for any unsupported field.

I agree the desirable behaviour is if an app correctly sets request_mask at most to XSTAT_REQUEST__ALL_STATS 0x00003fffULL (or whatever it is at the time the app is compiled that matches the current struct xstat), and if the kernel understands e.g. XSTAT_REQUEST_INO_HIGH or not is irrelevant since the kernel will not touch fields that are not requested. Likewise, if the application is compiled with a newer/larger XSTAT_REQUEST__ALL_STATS mask than what the kernel understands, the kernel will ignore the flags it doesn't understand.

Cheers, Andreas







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