util/iov: Do not assert offset is in iov

iov_from_buf(), iov_to_buf(), iov_memset(), and iov_copy() asserts
that the given offset fits in the iov while tolerating the specified
number of bytes to operate with to be greater than the size of iov.
This is inconsistent so remove the assertions.

Asserting the offset fits in the iov makes sense if it is expected that
there are other operations that process the content before the offset
and the content is processed in order. Under this expectation, the
offset should point to the end of bytes that are previously processed
and fit in the iov. However, this expectation depends on the details of
the caller, and did not hold true at least one case and required code to
check iov_size(), which is added with commit 83ddb3dbba
("hw/net/net_tx_pkt: Fix overrun in update_sctp_checksum()").

Adding such a check is inefficient and error-prone. These functions
already tolerate the specified number of bytes to operate with to be
greater than the size of iov to avoid such checks so remove the
assertions to tolerate invalid offset as well. They return the number of
bytes they operated with so their callers can still check the returned
value to ensure there are sufficient space at the given offset.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

util/iov.c

@@ -37,7 +37,6 @@ size_t iov_from_buf_full(const struct iovec *iov, unsigned int iov_cnt,
             offset -= iov[i].iov_len;
         }
     }
-    assert(offset == 0);
     return done;
 }
 
@@ -56,7 +55,6 @@ size_t iov_to_buf_full(const struct iovec *iov, const unsigned int iov_cnt,
             offset -= iov[i].iov_len;
         }
     }
-    assert(offset == 0);
     return done;
 }
 
@@ -75,7 +73,6 @@ size_t iov_memset(const struct iovec *iov, const unsigned int iov_cnt,
             offset -= iov[i].iov_len;
         }
     }
-    assert(offset == 0);
     return done;
 }
 
@@ -277,7 +274,6 @@ unsigned iov_copy(struct iovec *dst_iov, unsigned int dst_iov_cnt,
         bytes -= len;
         offset = 0;
     }
-    assert(offset == 0);
     return j;
 }
 
@@ -348,7 +344,6 @@ size_t qemu_iovec_concat_iov(QEMUIOVector *dst,
             soffset -= src_iov[i].iov_len;
         }
     }
-    assert(soffset == 0); /* offset beyond end of src */
 
     return done;
 }