exec: stash bpf-selected interpreter state in struct linux_binprm

The upcoming bpf-backed binfmt_misc handlers decide how a binary is run
programmatically at exec time: the interpreter itself, an optional
single argument to pass to it, and the invocation flags that a static
binfmt_misc entry fixes at registration time. The selection runs before
load_misc_binary() has copied the binary path from bprm->interp into
the argument vector, so the selecting program cannot go through
bprm_change_interp() directly without clobbering argv[1].

Stage the selected state in the bprm instead, grouped in struct
binfmt_misc_bpf and embedded anonymously in struct linux_binprm so the
bprm->bpf_* accesses stay direct. The bprm is exclusively owned by the
task doing the exec so no synchronization is needed. The consumers
free and clear the fields once the exec attempt that set them is
finished; free_bprm() covers all error paths.

Link: https://patch.msgid.link/20260714-work-bpf-binfmt_misc-v2-1-57b7529c002c@kernel.org
Reviewed-by: Farid Zakaria <farid.m.zakaria@gmail.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

fs/exec.c

@@ -1418,6 +1418,8 @@ static void free_bprm(struct linux_binprm *bprm)
 	/* If a binfmt changed the interp, free it. */
 	if (bprm->interp != bprm->filename)
 		kfree(bprm->interp);
+	kfree(bprm->bpf_interp);
+	kfree(bprm->bpf_interp_arg);
 	kfree(bprm->fdpath);
 	kfree(bprm);
 }

include/linux/binfmts.h

@@ -12,6 +12,13 @@ struct coredump_params;
 
 #define CORENAME_MAX_SIZE 128
 
+/* Interpreter selection staged by a bpf binfmt_misc handler. */
+struct binfmt_misc_bpf {
+	const char *bpf_interp;		/* interpreter selected by a bpf handler */
+	const char *bpf_interp_arg;	/* interpreter argument from a bpf handler */
+	u64 bpf_flags;			/* enum bpf_binprm_flags from a bpf handler */
+};
+
 /*
  * This structure is used to hold the arguments that are used when loading binaries.
  */
@@ -65,6 +72,7 @@ struct linux_binprm {
 				   of the time same as filename, but could be
 				   different for binfmt_{misc,script} */
 	const char *fdpath;	/* generated filename for execveat */
+	struct binfmt_misc_bpf;	/* bpf handler interpreter selection */
 	unsigned interp_flags;
 	int execfd;		/* File descriptor of the executable */
 	unsigned long exec;