[PATCH 11/21] binfmt_misc: add a static transparent flag 'T'

[Christian Brauner <brauner@kernel.org>]

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.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 Documentation/admin-guide/binfmt-misc.rst | 10 ++++++++++
 fs/binfmt_misc.c                          |  6 ++++++
 2 files changed, 16 insertions(+)

diff --git a/Documentation/admin-guide/binfmt-misc.rst b/Documentation/admin-guide/binfmt-misc.rst
index c76381a4bcad..c370c72c7dbe 100644
--- a/Documentation/admin-guide/binfmt-misc.rst
+++ b/Documentation/admin-guide/binfmt-misc.rst
@@ -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:
diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index 239cca389e0e..ed5ffb330749 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -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. */
@@ -763,6 +764,11 @@ 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)

-- 
2.53.0