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docs/ausearch-expression.5
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Steve Grubb
Fix parsing of virtual timestamp fields in ausearch_expression
28 мар 2018, 22:30
28 мар 2018, 22:304dcbcd8
.TH "AUSEARCH-EXPRESSION" "5" "Feb 2008" "Red Hat" "Linux Audit"
.SH NAME
ausearch-expression \- audit search expression format

.SH OVERVIEW
This man page describes the format of "ausearch expressions".
Parsing and evaluation of these expressions is provided by libauparse
and is common to applications that use this library.

.SH LEXICAL STRUCTURE

White space (ASCII space, tab and new-line characters) between tokens is
ignored.
The following tokens are recognized:

.TP
Punctuation
.B ( ) \e

.TP
Logical operators
.B ! && ||

.TP
Comparison operators
.B < <= == > >= !== i= i!= r= r!=

.TP
Unquoted strings
Any non-empty sequence of ASCII letters, digits, and the
.B _
symbol.

.TP
Quoted strings
A sequence of characters surrounded by the
.B \(dq
quotes.
The
.B \e
character starts an escape sequence.
The only defined escape sequences are
.B \e\e
and \fB\e\(dq\fR.
The semantics of other escape sequences is undefined.

.TP
Regexps
A sequence of characters surrounded by the
.B /
characters.
The
.B \e
character starts an escape sequence.
The only defined escape sequences are
.B \e\e
and \fB\e/\fR.
The semantics of other escape sequences is undefined.

.PP
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.

.SH EXPRESSION SYNTAX

The primary expression has one of the following forms:
.IP
.I field comparison-operator value

.B \eregexp
.I string-or-regexp
.PP

.I field
is either a string,
which specifies the first field with that name within the current audit record,
or the
.B \e
escape character followed by a string,
which specifies a virtual field with the specified name
(virtual fields are defined in a later section).

.I field
is a string.
.I operator
specifies the comparison to perform

.TP
.B r= r!=
Get the "raw" string of \fIfield\fR,
and compare it to \fIvalue\fR.
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
.BR auparse_get_field_str (3).
Each virtual field may define a "raw" string.
If
.I field
is not present or does not define a "raw" string,
the result of the comparison is
.B false
(regardless of the operator).

.TP
.B i= i!=
Get the "interpreted" string of \fIfield\fR,
and compare it to \fIvalue\fR.
For fields in audit records,
the "interpreted" string is an "user-readable" interpretation of the field
value;
applications can read the "interpreted" string using
.BR auparse_interpret_field (3).
Each virtual field may define an "interpreted" string.
If
.I field
is not present or does not define an "interpreted" string,
the result of the comparison is
.B false
(regardless of the operator).

.TP
.B < <= == > >= !==
Evaluate the "value" of \fIfield\fR, and compare it to \fIvalue\fR.
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 \fIvalue\fR for comparing it with the "value" of
.I field
are specific for each \fIfield\fR.
If
.I field
is not present,
the result of the comparison is
.B false
(regardless of the operator).
If
.I field
does not define a "value", an error is reported when parsing the expression.
.PP

In the special case of
.B \eregexp
\fIregexp-or-string\fR,
the current audit record is taken as a string
(without interpreting field values),
and matched against \fIregexp-or-string\fR.
.I regexp-or-string
is an extended regular expression, using a string or regexp token
(in other words, delimited by
.B \(dq
or \fB/\fR).

If
.I E1
and
.I E2
are valid expressions,
then
.B !
\fIE1\fR,
.I E1
.B &&
\fIE2\fR, and
.I E1
.B ||
.I E2
are valid expressions as well, with the usual C semantics and evaluation
priorities.
Note that
.B !
.I field op value
is interpreted as \fB!(\fIfield op value\fB)\fR, not as
\fB(!\fIfield\fB)\fI op value\fR.

.SH VIRTUAL FIELDS

The following virtual fields are defined:

.TP
.B \etimestamp
The value is the timestamp of the current event.
.I value
must be formatted as:
.sp
.in +5
.nf
.na
ts:seconds.milli
.ad
.fi
.in -5
.sp
where
.I seconds
and
.I milli
are decimal numbers specifying the seconds and milliseconds part of the
timestamp, respectively.

.TP
.B \etimestamp_ex
This is similar to
.B \etimestamp
but also includes the event's serial number.
.I value
must be formatted as:
.sp
.in +5
.nf
.na
ts:seconds.milli:serial
.ad
.fi
.in -5
.sp
where
.I serial
is a decimal number specifying the event's serial number.

.TP
.B \erecord_type
The value is the type of the current record.
.I value
is either the record type name, or a decimal number specifying the type.

.SH SEMANTICS
The expression as a whole applies to a single record.
The expression is
.B true
for a specified event if it is
.B true
for any record associated with the event.

.SH EXAMPLES

As a demonstration of the semantics of handling missing fields, the following
expression is
.B true
if
.I field
is present:
.IP
.B (\fIfield\fB r= \(dq\(dq) || (\fIfield\fB r!= \(dq\(dq)
.PP
and the same expression surrounded by
.B !(
and
.B )
is
.B true
if
.I field
is not present.

.SH 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 \fBfalse\fR.

New formats of value constants for the
.B \etimestamp
virtual field may be added.

.SH AUTHOR
Miloslav Trmac