Re: [PATCH 2/3] Introduce libfsverity

[Jes Sorensen <jes.sorensen@gmail.com>]

On 5/21/20 12:08 PM, Eric Biggers wrote:
> On Thu, May 21, 2020 at 11:24:34AM -0400, Jes Sorensen wrote:

>> Eric,
>>
>> Here is a more detailed review. The code as we have it seems to work for
>> me, but there are some issues that I think would be right to address:
> 
> Thanks for the feedback!
> 
>>
>> I appreciate that you improved the error return values from the original
>> true/false/assert handling.
>>
>> As much as I hate typedefs, I also like the introduction of
>> libfsverity_read_fn_t as function pointers are somewhat annoying to deal
>> with.
>>
>> My biggest objection is the export of kernel datatypes to userland and I
>> really don't think using u32/u16/u8 internally in the library adds any
>> value. I had explicitly converted it to uint32_t/uint16_t/uint8_t in my
>> version.
> 
> I prefer u64/u32/u16/u8 since they're shorter and easier to read, and it's the
> same convention that is used in the kernel code (which is where the other half
> of fs-verity is).

I like them too, but I tend to live in kernel space.

> Note that these types aren't "exported" to or from anywhere but rather are just
> typedefs in common/common_defs.h.  It's just a particular convention.
> 
> Also, fsverity-utils is already using this convention prior to this patchset.
> If we did decide to change it, then we should change it in all the code, not
> just in certain places.

I thought I did it everywhere in my patch set?

>> I also wonder if we should introduce an
>> libfsverity_get_digest_size(alg_nr) function? It would be useful for a
>> caller trying to allocate buffers to store them in, to be able to do
>> this prior to calculating the first digest.
> 
> That already exists; it's called libfsverity_digest_size().
> 
> Would it be clearer if we renamed:
> 
> 	libfsverity_digest_size() => libfsverity_get_digest_size()
> 	libfsverity_hash_name() => libfsverity_get_hash_name()

Oh I missed you added that. Probably a good idea to rename them for
consistency.

>>> diff --git a/lib/compute_digest.c b/lib/compute_digest.c
>>> index b279d63..13998bb 100644
>>> --- a/lib/compute_digest.c
>>> +++ b/lib/compute_digest.c
>>> @@ -1,13 +1,13 @@
>> ... snip ...
>>> -const struct fsverity_hash_alg *find_hash_alg_by_name(const char *name)
>>> +LIBEXPORT u32
>>> +libfsverity_find_hash_alg_by_name(const char *name)
>>
>> This export of u32 here is problematic.
> 
> It's not "exported"; this is a .c file.  As long as we use the stdint.h name in
> libfsverity.h (to avoid polluting the library user's namespace), it is okay.
> u32 and uint32_t are compatible; they're just different names for the same type.

I would still keep it consistent avoid relying on assumptions that types
are identical.

>>> +struct fsverity_signed_digest {
>>> +	char magic[8];			/* must be "FSVerity" */
>>> +	__le16 digest_algorithm;
>>> +	__le16 digest_size;
>>> +	__u8 digest[];
>>> +};
>>
>> I don't really understand why you prefer to manage two versions of the
>> digest, ie. libfsverity_digest and libfsverity_signed_digest, but it's
>> not a big deal.
> 
> Because fsverity_signed_digest has a specific endianness, people will access the
> fields directly and forget to do the needed endianness conventions -- thus
> producing code that doesn't work on big endian systems.  Using a
> native-endianness type for the intermediate struct avoids that pitfall.
> 
> I think keeping the byte order handling internal to the library is preferable.

Fair enough

>>> +static void *xmalloc(size_t size)
>>> +{
>>> +	void *p = malloc(size);
>>> +
>>> +	if (!p)
>>> +		libfsverity_error_msg("out of memory");
>>> +	return p;
>>> +}
>>> +
>>> +void *libfsverity_zalloc(size_t size)
>>> +{
>>> +	void *p = xmalloc(size);
>>> +
>>> +	if (!p)
>>> +		return NULL;
>>> +	return memset(p, 0, size);
>>> +}
>>
>> I suggest we get rid of xmalloc() and libfsverity_zalloc(). libc
>> provides perfectly good malloc() and calloc() functions, and printing an
>> out of memory error in a generic location doesn't tell us where the
>> error happened. If anything those error messages should be printed by
>> the calling function IMO.
>>
> 
> Maybe.  I'm not sure knowing the specific allocation sites would be useful
> enough to make all the callers handle printing the error message (which is
> easily forgotten about).  We could also add the allocation size that failed to
> the message here.

My point is mostly at this point, this just adds code obfuscation rather
than adding real value. I can see how it was useful during initial
development.

Cheers,
Jes