如何排查Android中my_tool引发的dac_override问题及含义疑问？

Hey there! Let's tackle your two questions one by one—first confirming what dac_override means, then walking through how to track down the exact operation and file your my_tool is trying to access.

First: Confirming dac_override's Meaning #

Your guess is spot-on! Let's formalize it clearly:

• DAC stands for Discretionary Access Control—this is the classic Linux file permission model you see with ls -l: the read/write/execute bits assigned to the file's owner, group, and all other users.
• dac_override is an SELinux permission that governs whether a process can bypass those traditional DAC permissions. When your my_tool triggers this, it means the app tried to perform an action (like reading a file it doesn't have r rights for, or writing to a restricted directory) that would normally be blocked by standard Linux permissions. SELinux then checks if the process has the dac_override permission to get around that block.

Second: Tracking Down the Exact Operation & Target File #

Here's a step-by-step workflow to pinpoint what's happening:

1. Extract Detailed Context from dmesg #

First, filter your dmesg logs to focus on events related to my_tool or dac_override:

dmesg | grep -E "(dac_override|my_tool)"

SELinux denial messages (the ones triggering dac_override checks) will usually include all the details you need:

• The specific operation (e.g., open, write, chown)
• The full path to the target file/directory
• The PID and SELinux context of my_tool
• The SELinux context of the target object

For example, a log line might look like:

avc: denied { write } for pid=4567 comm="my_tool" path="/data/system/some_config.xml" dev="dm-1" ino=12345 scontext=u:r:my_tool:s0 tcontext=u:object_r:system_data_file:s0 tclass=file permissive=0
This directly tells you my_tool tried to write to /data/system/some_config.xml but lacked the traditional DAC permissions to do so.

2. Use audit2allow for Full Audit Log Parsing #

If dmesg doesn't show the full picture, temporarily switch SELinux to permissive mode to capture all audit events:

setenforce 0

Run my_tool as you normally would, then run this command to parse the audit logs:

audit2allow -a

This tool will break down exactly what operations my_tool attempted, which files it targeted, and why the DAC permission check failed.

3. Trace System Calls with strace #

To see every file-related action my_tool takes right before the dac_override trigger, use strace to log file-focused system calls:

strace -f -e trace=file ./my_tool

Look for lines where the system call returns -1 EPERM (permission denied)—that's the exact point where my_tool hits a DAC permission wall, triggering the SELinux check.

4. Verify Permissions & Contexts #

Once you have a suspected target file, confirm its traditional DAC permissions:

ls -l /path/to/target/file

And check its SELinux context to rule out additional context mismatches:

ls -Z /path/to/target/file

You can also check which user my_tool is running as with:

ps -u | grep my_tool

This will confirm if the running user lacks the necessary read/write/execute bits on the target file.

内容的提问来源于stack exchange，提问作者satur9nine