TEXTDUMP(4)

💾 TEXTDUMP(4) 💾

okay bestie, gather round, because this one is the Kernel Interfaces Manual page for textdump — the textdump kernel dumping facility — and it is genuinely one of the most underrated little divas in the whole tree. no cap. 🫶

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✨ NAME ✨

textdump — textdump kernel dumping facility

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🌸 SYNOPSIS 🌸

options DDB
options KDB

options TEXTDUMP_PREFERRED
options TEXTDUMP_VERBOSE

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🦋 DESCRIPTION 🦋

so like. when your kernel does a little oopsie and needs to spill its guts, the normal move is a memory dump or minidump — very machine-readable, very "here's a wall of raw bytes, good luck boo". The textdump facility says not today and instead captures kernel debugging information to disk in a human-readable form instead. 💅

and yeah, this representation is less complete — it doesn't capture the full kernel state, we're not pretending otherwise — but the tradeoff is chef's kiss: it gives you debugging info in a form that's more compact, more portable, and more persistent than a traditional dump. 🎀

and here's the iconic part: by combining textdump with other ddb(4) facilities — think scripting and output capture — you can capture detailed bug information in a fully automated manner. it's giving main character energy fr fr. ✨

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🎀 FORMAT 🎀

textdump data is stored in a dump partition in the same style as a regular memory dump, and it will be automatically extracted by savecore(8) if present on boot. automatic. we love a self-sufficient queen. 👑

textdump files are stored in the tar(5) format, and they consist of one or more text files, each one storing a particular type of debugging output. the following parts may be present:

• ddb.txt — Captured ddb(4) output, if the capture facility has been used. May be disabled by clearing the debug.ddb.textdump.do_ddb sysctl.
• config.txt — Kernel configuration, if options INCLUDE_CONFIG_FILE has been compiled into the kernel. May be disabled by clearing the debug.ddb.textdump.do_config sysctl.
• msgbuf.txt — Kernel message buffer, including recent console output if the capture facility has been used. May be disabled by clearing the debug.ddb.textdump.do_msgbuf sysctl.
• panic.txt — Kernel panic string, if the kernel panicked before the dump was generated. May be disabled by clearing the debug.ddb.textdump.do_panic sysctl.
• version.txt — Kernel version string. My be disabled by clearing the debug.ddb.textdump.do_version sysctl.

(and yes, that "My be disabled" typo in version.txt is in the original, we're keeping it, it's part of the lore now 🙈)

Kernel textdumps may be extracted using tar(1). 🧚

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💜 CONFIGURATION 💜

The textdump facility is enabled as part of the kernel debugger using options KDB and options DDB.

By default, kernel dumps generated on panic or via explicit requests for a dump will be regular memory dumps. however — plot twist — by using the textdump set command in ddb(4), or by setting the debug.ddb.textdump.pending sysctl to 1 using sysctl(8), you can request that the next dump be a textdump instead. you can also directly trigger a textdump in ddb(4) by running the command textdump dump. periodt. 💅

If you're at the ddb(4) command line, the commands textdump set, textdump status, and textdump unset may be used to set, query, and clear the textdump pending flag. set, status, unset — the holy trinity bestie. 🫶

As with regular kernel dumps, a dump partition must be automatically or manually configured using dumpon(8).

Additional kernel config(8) options:

• TEXTDUMP_PREFERRED — sets textdumps to be the default manner of doing dumps. This means there will be no need to sysctl(8) or use the textdump set ddb(8) commands. she just does it by default now, iconic behaviour honestly 🌈
• TEXTDUMP_VERBOSE — will have the textdump facility be more verbose about each file it is emitting, as well as other diagnostics useful to debug the textdump facility itself. yapper mode: enabled 🗣️✨

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🔥 EXAMPLES 🔥

In the following example, the script kdb.enter.panic will run when the kernel debugger is entered as a result of a panic, enable output capture, dump several useful pieces of debugging information, and then invoke panic in order to force a kernel dump to be written out followed by a reboot:

script kdb.enter.panic=textdump set; capture on; show allpcpu; bt;
  ps; alltrace; show alllocks; textdump dump; reset

In the following example, the script kdb.enter.witness will run when the kernel debugger is entered as a result of a witness violation, printing lock-related information for the user:

script kdb.enter.witness=show locks

These scripts may also be configured using the ddb(8) utility. 🛠️

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👀 SEE ALSO 👀

tar(1), ddb(4), tar(5), ddb(8), dumpon(8), savecore(8), sysctl(8)

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📜 HISTORY 📜

The textdump facility first appeared in FreeBSD 7.1.

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💖 AUTHORS 💖

The textdump facility was created by Robert N. M. Watson.

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lowkey obsessed with whoever decided kernel debugging info deserved to be readable. go off king 🫡

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and that's the whole page bestie!! this lil baddie is coming to you straight from the Debian build of the manual, dated October 18, 2019. go forth and textdump set responsibly. mwah 💋🌸