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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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36
kernel/sys/sys_fs.c
View file

@ -987,34 +987,20 @@ addr_t sys_unlink(void)
addr_t sys_exec(void)
{
char *path, *argv[MAXARG], *envp[MAXENV];
addr_t uargv, uenvp, uarg;
char *path;
addr_t uargv, uenvp;
if (argstr(0, &path) < 0 || argaddr(1, &uargv) < 0) return -EFAULT;
memset(argv, 0, sizeof(argv));
for (int i = 0; ; i++) {
if (i >= NELEM(argv)) return -E2BIG;
if (fetchaddr(uargv + sizeof(addr_t) * i, &uarg) < 0) return -EFAULT;
if (!uarg) { argv[i] = 0; break; }
if (fetchstr(uarg, &argv[i]) < 0) return -EFAULT;
}
// The third argument. This is Linux's execve(2), so musl has been passing
// an environment here all along -- it was simply never read, and every
// program on the system started with an empty one.
// The arrays are read where they are, by exec_user(), rather than copied
// into fixed arrays here: there used to be room for 32 arguments and 64
// variables, and `rm *` in a directory of 40 files failed with E2BIG.
//
// A null envp is legal and means exactly that. It is also what the
// first-party exec() wrapper passes, so old callers keep their old
// behaviour without knowing about this.
memset(envp, 0, sizeof(envp));
if (argaddr(2, &uenvp) == 0 && uenvp) {
for (int i = 0; ; i++) {
if (i >= NELEM(envp)) return -E2BIG;
if (fetchaddr(uenvp + sizeof(addr_t) * i, &uarg) < 0) return -EFAULT;
if (!uarg) { envp[i] = 0; break; }
if (fetchstr(uarg, &envp[i]) < 0) return -EFAULT;
}
}
return exec(path, argv, envp);
// envp is Linux's third argument, which musl had been passing all along
// while nothing read it. A null envp is legal and means "none"; it is also
// what the first-party exec() wrapper passes.
if (argaddr(2, &uenvp) < 0)
uenvp = 0;
return exec_user(path, uargv, uenvp);
}
// ftruncate/truncate both grow and shrink. Growing used to be quietly dropped