if_firewire: Be more robust while checking the frame size

[zlei]

Summary

The type of m_pkthdr.len is int. In case it is negative, say bugs from
driver ( although that should be rare ), the comparing to size_t will
promote the negative int to size_t to a large value, and the result
is likely to be the opposite which is obviously not expected.

While here, free the mbuf chain and return immediately if the frame is
invalid.

MFC after: 1 week

[zlei]

sys/net/if_fwsubr.c

522

@glebius This is the pitfall of C language. I do not like very much the implicit integer promotion from int to unsigned. That is the source of bugs.

Clang has an option -Wsign-compare but that results in lots of false positive reports which is not very useful.

Any suggestions ?

[adrian]

is m_len / m_pktlen ever supposed to be negative?

[zlei]

In D61320#1405672, @adrian wrote:

is m_len / m_pktlen ever supposed to be negative?

They should not be negative. But buggy driver / net stack logic may lead to. Here is an example 6429199a2113 (llan: byte swap the receive queue entries) .

[adrian]

In D61320#1405753, @zlei wrote:

In D61320#1405672, @adrian wrote:

is m_len / m_pktlen ever supposed to be negative?

They should not be negative. But buggy driver / net stack logic may lead to. Here is an example 6429199a2113 (llan: byte swap the receive queue entries) .

If it can't be negative then we could write a mbuf len comparison function that can assert/log an error, and then do the correct things with comparison.