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binfmt_misc: add a static transparent flag 'T'

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Let a registration opt into transparent dispatch. The 'T' flag lets a
matched binary keep its argument vector and is sent to the interpreter
through AT_EXECFD. The process's identity is the binary's.

'T' implies 'O' exactly like 'C' does. 'P' is rejected in combination
with it. Transparency preserves the whole argument vector so there is
nothing left for 'P' to say. 'C' remains an independent choice and 'F'
keeps working. A pre-opened interpreter is orthogonal to how the binary
is handed over.

Like the other flag characters 'T' cannot be used as the field
delimiter. The flag scan would run off the registration buffer.

Link: https://patch.msgid.link/20260721-work-bpf-binfmt_misc-ptinterp-v2-11-e57866e4ae0f@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
This commit is contained in:
Christian Brauner 2026-07-21 16:13:53 +02:00
parent a4bdab2be4
commit 75e536852f
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2 changed files with 16 additions and 0 deletions
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10
Documentation/admin-guide/binfmt-misc.rst
View File

@ -90,6 +90,16 @@ Here is what the fields mean:
emulation is installed and uses the opened image to spawn the
emulator, meaning it is always available once installed,
regardless of how the environment changes.
``T`` - transparent
Run the interpreter transparently. The binary is handed to
the interpreter through ``AT_EXECFD`` (``T`` implies ``O``),
the argument vector is left exactly as the caller built it
and the kernel labels ``/proc/pid/exe`` with the binary
instead of the interpreter. The interpreter has to load the
binary from ``AT_EXECFD`` and follow the
``AT_FLAGS_TRANSPARENT_INTERP`` contract. Combining ``T``
with ``P`` is rejected: transparency preserves the whole
argument vector, argv[0] included.
There are some restrictions:

6
fs/binfmt_misc.c
View File

@ -72,6 +72,7 @@ static const struct binfmt_misc_flag misc_flags[] = {
{ 'O', MISC_FMT_OPEN_BINARY, 0, "open binary" },
{ 'C', MISC_FMT_CREDENTIALS, MISC_FMT_OPEN_BINARY, "credentials from the binary" },
{ 'F', MISC_FMT_OPEN_FILE, 0, "open interpreter file now" },
{ 'T', MISC_FMT_TRANSPARENT, MISC_FMT_OPEN_BINARY, "transparent" },
};
/* Look up a flag character, NULL if @c is not one. */
@ -764,6 +765,11 @@ static struct binfmt_misc_entry *create_entry(const char __user *buffer,
if (test_bit(MISC_FMT_BPF_BIT, &e->flags) && p != flags)
return ERR_PTR(-EINVAL);
/* Transparency preserves the whole argv, argv[0] included. */
if ((e->flags & MISC_FMT_TRANSPARENT) &&
(e->flags & MISC_FMT_PRESERVE_ARGV0))
return ERR_PTR(-EINVAL);
if (*p == '\n')
p++;
if (p != buf + count)