| Filename | Latest commit message | Latest commit date |
|---|---|---|
51983144e3
exec took 32 arguments and 64 environment variables, in fixed arrays, so a glob over more than 31 files failed with E2BIG. It now reads the caller's argv and envp where they are, through the uaccess helpers and a bounce page -- a second thread rewriting the array after a check cannot slip a kernel address in -- and sizes the new stack to fit them. The limits are Linux's: ARG_MAX, 2 MB of strings and pointers, and 128 KB for any one string (USER_STR_MAX follows it). Every mmap took one of a process's 256 VMA slots, and zsh takes one per 16 KB heap block, so `x=($(<file))` over 30000 words ran out after 4 MB of heap and exited 1 without a word. Neighbouring anonymous mappings now share a record, as Linux merges them -- the bump allocator puts each new one where the last ended -- and munmap trims or splits it. Partial unmaps of device or segment mappings are refused with EINVAL instead of kfree()ing pages that are not the process's, and a failed allocation half way through a mapping no longer leaks what it had. Checked under QEMU: touch over 3000 names, /bin/echo with 50000 arguments, E2BIG past 2 MB, zsh splitting 100000 words, fork loops. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4 |
||
| .. | ||
| kernel.md | ||
| moving-to-linux.md | ||
| README.md | ||
| schfs.md | ||
History
Why SchOS is the way it is. Bugs whose cause was nowhere near their symptom, and the few write-ups that have nowhere better to live.
| kernel.md | The expensive ones. Boot, SMP, signals, ABI assumptions, /proc, input. |
| schfs.md | How the filesystem was built, and what each phase cost. |
| moving-to-linux.md | The Windows-to-Fedora move. |
What belongs in here
A bug whose cause is not visible from the code that fixed it.
That is the whole test. If reading the fix explains the problem, the fix is the documentation and nothing needs writing down. If the fix looks arbitrary — a constant moved by eight, a field reordered, a check inverted — then the strangeness is load-bearing and the next person to touch it deserves to know why before they change it back.
Every entry here is a thing that looked like a different problem than it was.
What does not belong
- A changelog.
git logis the changelog. - A bug that was found and fixed in an hour. Cost is the filter.
- How something works. That is ../architecture/, written in the present tense for somebody who does not care what it used to be.
- Anything still open. An unfinished investigation goes in ../plan/ or in the code as a comment. This directory is past tense.
How to write one
Symptom first, then diagnosis, then the fix, then the transferable part.
The transferable part is the reason the entry exists. __thread being wrong for
per-CPU data is worth more than the specific null dereference it caused;
"a layout constant that several artifacts must agree on wants a check that stops
the build, not a comment asking people to remember" is worth more than the
bootloader offset that taught it.
Say how it was found, when the technique was the hard part. A hardware
watchpoint through QEMU's gdb stub named an instruction on the first run after
three rounds of cprintf had narrowed it to nothing — that is reusable, and the
bug is not.
Do not clean up the embarrassing part. The near-miss where a change passed
every assertion green without having been applied at all is one of the most
useful entries in this directory, and it only works because it admits that a
grep returning three hits was read as confirmation without counting what they
were.