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exec: Linux's 2 MB of arguments; mmap: merge neighbouring mappings

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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
This commit is contained in:
Nexoniarz 2026-10-23 13:20:29 +02:00
commit 51983144e3
7 changed files with 366 additions and 133 deletions
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7
include/param.h
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@ -42,8 +42,11 @@
#else
#define ROOTDEV 1 // device number of file system root disk
#endif
#define MAXARG 32 // max exec arguments
#define MAXENV 64 // max environment variables passed through exec
// exec's limits, Linux's: ARG_MAX bytes of argument and environment strings
// and their pointers together, and no one string over ARG_STRLEN_MAX (Linux's
// MAX_ARG_STRLEN). There is no limit on the count beyond that.
#define ARG_MAX (2 * 1024 * 1024)
#define ARG_STRLEN_MAX (128 * 1024)
#define MAXPATH 128 // max path
#define MAXOPBLOCKS 30 // max # of blocks any FS op writes
#define LOGSIZE (MAXOPBLOCKS*3) // max data blocks in on-disk log