[PATCH] uapi: increase MAX_ARG_STRLEN from 128K to 6M

[Sergei Trofimovich]

Before the change linux allowed individual execve() arguments or
environment variable entries to be only as big as 32 pages.

Histroically before b6a2fea3931 "mm: variable length argument support"
MAX_ARG_STRLEN used to be full allowed size `argv[] + envp[]`.

When full limit was abandoned individual parameters were still limited
by a safe limit of 128K.

Nowadays' linux allows `argv[]+envp[]` to be as laerge as 6MB (3/4
`_STK_LIM`).

Some build systems like `autoconf` use a single environment variable
to pass `CFLAGS` environment variable around. It's not a bug problem
if the argument list is short.

But some packaging systems prefer installing each package into
individual directory. As a result that requires quite long string of
parameters like:

    CFLAGS="-I/path/to/pkg1 -I/path/to/pkg2 ..."

This can easily overflow 128K and does happen for `NixOS` and `nixpkgs`
repositories on a regular basis.

Similar pattern is exhibited by `gcc` which converts it's input command
line into a single environment variable (https://gcc.gnu.org/PR111527):

  $ big_100k_var=$(printf "%0*d" 100000 0)

  # this works: 200KB of options for `printf` external command
  $ $(which printf) "%s %s" $big_100k_var $big_100k_var >/dev/null; echo $?
  0

  # this fails: 200KB of options for `gcc`, fails in `cc1`
  $ touch a.c; gcc -c a.c -DA=$big_100k_var -DB=$big_100k_var
  gcc: fatal error: cannot execute 'cc1': execv: Argument list too long
  compilation terminated.

I would say this 128KB limitation is arbitrary.
The change raises the limit of `MAX_ARG_STRLEN` from 32 pakes (128K
n `x86_64`) to the maximum limit of stack allowed by Linux today.

It has a minor chance of overflowing userspace programs that use
`MAX_ARG_STRLEN` to allocate the strings on stack. It should not be a
big problem as such programs are already are at risk of overflowing
stack.

Tested as:
    $ V=$(printf "%*d" 1000000 0) ls

Before the change it failed with `ls: Argument list too long`. After the
change `ls` executes as expected.

WDYT of abandoning the limit and allow user to fill entire environment
with a single command or a single variable?

CC: Eric Biederman <ebiederm@xxxxxxxxxxxx>
CC: Kees Cook <keescook@xxxxxxxxxxxx>
CC: linux-mm@xxxxxxxxx
CC: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Sergei Trofimovich <slyich@xxxxxxxxx>
---
 include/uapi/linux/binfmts.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/uapi/linux/binfmts.h b/include/uapi/linux/binfmts.h
index c6f9450efc12..4e828515a22e 100644
--- a/include/uapi/linux/binfmts.h
+++ b/include/uapi/linux/binfmts.h
@@ -8,11 +8,11 @@ struct pt_regs;
 
 /*
  * These are the maximum length and maximum number of strings passed to the
- * execve() system call.  MAX_ARG_STRLEN is essentially random but serves to
- * prevent the kernel from being unduly impacted by misaddressed pointers.
+ * execve() system call.  MAX_ARG_STRLEN is as large as Linux allows new
+ * stack to grow.  Currently it's `_STK_LIM / 4 * 3 = 6MB` (see fs/exec.c).
  * MAX_ARG_STRINGS is chosen to fit in a signed 32-bit integer.
  */
-#define MAX_ARG_STRLEN (PAGE_SIZE * 32)
+#define MAX_ARG_STRLEN (6 * 1024 * 1024)
 #define MAX_ARG_STRINGS 0x7FFFFFFF
 
 /* sizeof(linux_binprm->buf) */
-- 
2.42.0