What value gets stored in a3 when -S execve?

[Vishnu Srinivasa Ramaprasad <srvishnu123@gmail.com>]

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Hi Steve Grubb and Linux-Audit team,


I'm Vish and I am a newbie to auditd. My requirement is to log only
shell/bash commands and custom commands executed by administrator users.
I have created these rules in /etc/audit/rules.d/audit.rules, to ensure
SYSCALL, EXECVE are being added to audit.log for administrator users with
auid greater than 1000:
-a exit,always -F arch=b64 -S execve -F auid>=1000 -F auid!=-1 -k log_cmd
-a exit,always -F arch=b32 -S execve -F auid>=1000 -F auid!=-1 -k log_cmd

After restarting auditd service, I had executed strace on a simple command
which will display version of my project's toolkit:

~# strace -e trace=execve toolkit-version-show
execve("/var/tmp/toolkit-version-show", ["toolkit-version-show"],
0x7ffef1fa38b0 /* 30 vars */) = 0
Toolkit Version: 1.01
+++ exited with 0 +++

Later, I executed the ausearch command to check the log entry:
~# ausearch -i --start recent
----
type=EXECVE msg=audit(05/27/2020 19:01:26.605:12725) : argc=2
a0=/usr/bin/perl a1=/var/tmp/toolkit-version-show
type=SYSCALL msg=audit(05/27/2020 19:01:26.605:12725) : arch=x86_64
syscall=execve success=yes exit=0 a0=0x7ffef1fa2450 a1=0x7ffef1fa38a0
a2=0x7ffef1fa38b0 a3=0x7f47f8669740 items=3 ppid=3641 pid=3643
auid=administrator uid=root gid=root euid=root suid=root fsuid=root
egid=root sgid=root fsgid=root tty=pts0 ses=8936 comm=toolkit-version-sho
exe=/usr/bin/perl key=log_cmd
----
type=EXECVE msg=audit(05/27/2020 19:01:26.601:12724) : argc=4 a0=strace
a1=-e a2=trace=execve a3=toolkit-version-show
type=SYSCALL msg=audit(05/27/2020 19:01:26.601:12724) : arch=x86_64
syscall=execve success=yes exit=0 a0=0x55a2d44c9010 a1=0x55a2d449fe80
a2=0x55a2d4389490 a3=0x8 items=2 ppid=3099 pid=3641 auid=administrator
uid=root gid=root euid=root suid=root fsuid=root egid=root sgid=root
fsgid=root tty=pts0 ses=8936 comm=strace exe=/usr/bin/strace key=log_cmd
----

My understanding of a0 - a2: Please refer the syntax of execve() :

int execve(const char*filename, char *const argv[], char *const envp[]);

Based on syntax of execve() and output from strace, I could understand that
in audit.log event entry:
a0=0x7ffef1fa2450 represents filename argument of execve
a1=0x7ffef1fa38a0 represents argv[] argument of execve
a2=0x7ffef1fa38b0 represents envp[] argument of execve

Question  1:  What does the a3=0x7f47f8669740  value represent?
As execve() has only 3 arguments (represented by a0,a1,a2), what value gets
stored in a3?
I have noticed a3 values to be varying from:
a3=0x0
a3=0x7
a3=0x55a2d4389490
a3=0x56a2d44adc00
a3=0x8

Question 2: a3=0x8 seems to be the value assigned for a majority of
execve() syscalls.
Is this a standard value set in case of main/primary system call, such as
toolkit-version-show?
Could I use this in a rule filter *-F a3=8* to log only primary (custom
script) command executed by user and not internal commands executed by
custom script like ls, cat, grep.. etc.:
-a exit,always -F arch=b64 -S execve *-F a3=8* -F auid>=1000 -F auid!=-1 -k
log_cmd
-a exit,always -F arch=b32 -S execve *-F a3=8* -F auid>=1000 -F auid!=-1 -k
log_cmd

Would I miss logging a few primary syscalls, if a3 is not 0x80 in some
cases?

Question 3: If a3=0x8 is not a standard value, Is it possible to identify
primary custom command and log only that command, and not internal commands
with a3=0x0 or a3=0x55a2d4389490?

Question 4: Is it possible to filter out and not log syscalls with
tty=(none)?

I am looking forward to your kind response.

Thank You
Regards,
Vish

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