peekfd

peekfd [options] PID FD [offset] [length]

Where:
PID is the Process ID of the target application.
FD is the file descriptor number within the target process.
offset (optional) is the byte offset from the current position of the file descriptor where reading should begin. If omitted, it typically starts from the current file position.
length (optional) is the number of bytes to read. If omitted, a default length (e.g., 4KB) or reading until EOF might be applied.

-x
    Display output in hexadecimal format.

-a
    Display output in ASCII format, attempting to show printable characters.

-s
    Attempt to display output as a string, often stopping at the first null byte.

-o
    Redirect the peeked data to a specified output file instead of standard output.

-p
    Pause the target process while peeking (common for ptrace-based implementations).

-v
    Enable verbose output, showing more details about the operation.

peekfd is a specialized Linux debugging utility designed to inspect the contents of a file descriptor belonging to another running process. Unlike commands like lsof which reveal information about file descriptors (what they point to), peekfd allows a user to read the actual data that is currently associated with a specified file descriptor at a particular offset.

This can be invaluable for diagnosing issues in inter-process communication (IPC), such as pipes or sockets, or for observing data written to temporary files or logs by a process that is not yet consumed or flushed. The tool typically works by utilizing low-level kernel interfaces like the /proc filesystem or the ptrace system call, allowing it to attach to the target process and read from its memory space or inject read syscalls without affecting the process's normal execution flow significantly (though ptrace can briefly pause the target). Due to its nature, peekfd often requires root privileges or specific capabilities to function. It is important to note that peekfd is not a standard utility found in all Linux distributions; it is frequently implemented as a custom script or a small C program by developers for specific debugging needs.

peekfd operates at a low level and comes with several important considerations:
1. Non-Standard Tool: peekfd is not a universally distributed command; its availability, syntax, and functionality vary significantly between custom implementations. Users may need to compile it from source or use a similar custom tool.
2. Permissions: Reading file descriptors of other processes typically requires root privileges or the CAP_SYS_PTRACE capability, as it involves accessing kernel data structures or attaching to other processes.
3. Race Conditions: The data seen by peekfd can be subject to race conditions. The target process might read from or write to the file descriptor immediately before or after peekfd performs its operation, leading to data that is momentarily outdated.
4. Impact on Target Process: While designed to be non-intrusive, ptrace-based implementations might briefly pause the target process, which can have a minor performance impact or cause a slight delay.
5. File Descriptor Position: Some implementations might reset or alter the file descriptor's current read/write position (lseek) in the target process during the peek operation, which could subtly affect the target process's subsequent operations. However, well-designed tools aim to restore the original position.

peekfd typically employs one of two primary methods:
1. Ptrace-based: The most common approach involves using the ptrace system call. The peekfd process attaches to the target PID, effectively taking control. It can then either inject a read system call into the target process's context, causing the target to read its own file descriptor into a controlled memory region, or directly read memory pages from the target process's address space where file descriptor buffers might reside. After reading, ptrace detaches or allows the target to resume.
2. /proc Filesystem (less common for content): While /proc/<PID>/fd/ gives symlinks to the underlying files/devices, and /proc/<PID>/fdinfo/ provides metadata like offset and flags, directly reading the content from a file descriptor of another process via /proc isn't as straightforward as ptrace. Some specialized kernel modules or FUSE filesystems could hypothetically expose this, but standard peekfd implementations usually rely on ptrace.

• Debugging Pipes/Sockets: Observing data flowing through pipes or network sockets between processes without disrupting communication.
• Analyzing Buffered I/O: Checking what data is held in kernel or application buffers for a specific file descriptor before it's processed or flushed.
• Forensics/Troubleshooting: Gaining insights into a misbehaving process's data handling in real-time.

The concept of inspecting another process's file descriptor content stems from the need for advanced debugging of complex system interactions, particularly in distributed systems, network services, or highly concurrent applications. While a standardized peekfd command is not part of mainstream Linux distributions, the underlying techniques (like ptrace for process control and memory inspection, and the /proc filesystem for process state) have been available in Linux since its early days. Developers and system administrators have historically created custom tools or scripts leveraging these kernel features to achieve the peekfd functionality, often sharing them within specific communities or incorporating them into larger debugging frameworks. The tool's sporadic appearance reflects a niche but critical debugging requirement that isn't fully met by more general-purpose utilities.

lsof(8), strace(1), readlink(1), gdb(1)