1 p.m.
Hello,
this patch extends libauparse to support arbitrary boolean expressions
and searching for record type ranges.
It seems best to define expression syntax and add a single
ausearch_add_expression() call, rather than build the expression from
atomic subexpressions. The defined syntax could be shared by all tools
that link to libauparse, and programs would be able to run (and display
meaningful error messages, as opposed to crashing when a relocation can
not be resloved) even if the installed libauparse version does not
support the used filter.
Attached is a description of the expression syntax. Please check
whether the design is reasonably future-proof. A nicer alternative to
using three different sets of comparison operators would be welcome.
Mirek
LEXICAL STRUCTURE
White space (ASCII space, tab and new-line characters) between tokens
is ignored. The following tokens are recognized:
Punctuation
( ) \
Logical operators
! && ||
Comparison operators
< <= == > >= !== i= i!= r= r!=
Unquoted strings
Any non-empty sequence of ASCII letters, digits, and the _ sym-
bol.
Quoted strings
A sequence of characters surrounded by the " quotes. The \
character starts an escape sequence. The only defined escape
sequences are \\ and \". The semantics of other escape
sequences is undefined.
Anywhere an unquoted string is valid, a quoted string is valid as well,
and vice versa. In particular, field names may be specified using
quoted strings, and field values may be specified using unquoted
strings.
EXPRESSION SYNTAX
The primary expression has the following form:
field comparison-operator value
field is either a string, which specifies the first field with that
name within the current audit record, or the \ escape character fol-
lowed by a string, which specifies a virtual field with the specified
name (virtual fields are defined in a later section).
field is a string. operator specifies the comparison to perform
r= r!= Get the "raw" string of field, and compare it to value. For
fields in audit records, the "raw" string is the exact string
stored in the audit record (with all escaping and unprintable
character encoding left alone); applications can read the "raw"
string using auparse get field str(3). Each virtual field may
define a "raw" string. If field is not present or does not
define a "raw" string, the result of the comparison is false
(regardless of the operator).
i= i!= Get the "interpreted" string of field, and compare it to value.
For fields in audit records, the "interpreted" string is an
"user-readable" interpretation of the field value; applications
can read the "interpreted" string using auparse inter-
pret field(3). Each virtual field may define an "interpreted"
string. If field is not present or does not define an "inter-
preted" string, the result of the comparison is false (regard-
less of the operator).
< <= == > >= !==
Evaluate the "value" of field, and compare it to value. A
"value" may be defined for any field or virtual field, but no
"value" is currently defined for any audit record field. The
rules of parsing value for comparing it with the "value" of
field are specific for each field. If field is not present, the
result of the comparison is false (regardless of the operator).
If field does not define a "value", an error is reported when
parsing the expression.
If E1 and E2 are valid expressions, then ! E1, E1 && E2, and E1 || E2
are valid expressions as well, with the usual C semantics and evalua-
tion priorities. Note that ! field op value is interpreted as !(field
op value), not as (!field) op value.
VIRTUAL FIELDS
The following virtual fields are defined:
\timestamp
The value is the timestamp of the current event. value must
have the ts:seconds.milli format, where seconds and milli are
decimal numbers specifying the seconds and milliseconds part of
the timestamp, respectively.
\record_type
The value is the type of the current record. value is either
the record type name, or a decimal number specifying the type.
SEMANTICS
The expression as a whole applies to a single record. The expression
is true for a specified event if it is true for any record associated
with the event.
EXAMPLES
As a demonstration of the semantics of handling missing fields, the
following expression is true if field is present:
(field r= "") || (field r!= "")
and the same expression surrounded by !( and ) is true if field is not
present.
FUTURE DIRECTIONS
New escape sequences for quoted strings may be defined.
For currently defined virtual fields that do not define a "raw" or
"interpreted" string, the definition may be added. Therefore, don’t
rely on the fact that comparing the "raw" or "interpreted"
string of
the field with any value is false.
New formats of value constants for the \timestamp virtual field may be
added.