binfmt_misc: pass binfmt_misc flags to the interpreter

It can be useful to the interpreter to know which flags are in use.

For instance, knowing if the preserve-argv[0] is in use would
allow to skip the pathname argument.

This patch uses an unused auxiliary vector, AT_FLAGS, to add a
flag to inform interpreter if the preserve-argv[0] is enabled.

Note by Helge Deller:
The real-world user of this patch is qemu-user, which needs to know
if it has to preserve the argv[0]. See Debian bug #970460.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: YunQiang Su <ysu@wavecomp.com>
URL: http://bugs.debian.org/970460
Signed-off-by: Helge Deller <deller@gmx.de>

Author: vivier

fs/binfmt_elf.c

@@ -186,6 +186,7 @@ create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
 	unsigned char k_rand_bytes[16];
 	int items;
 	elf_addr_t *elf_info;
+	elf_addr_t flags = 0;
 	int ei_index;
 	const struct cred *cred = current_cred();
 	struct vm_area_struct *vma;
@@ -260,7 +261,9 @@ create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
 	NEW_AUX_ENT(AT_PHENT, sizeof(struct elf_phdr));
 	NEW_AUX_ENT(AT_PHNUM, exec->e_phnum);
 	NEW_AUX_ENT(AT_BASE, interp_load_addr);
-	NEW_AUX_ENT(AT_FLAGS, 0);
+	if (bprm->interp_flags & BINPRM_FLAGS_PRESERVE_ARGV0)
+		flags |= AT_FLAGS_PRESERVE_ARGV0;
+	NEW_AUX_ENT(AT_FLAGS, flags);
 	NEW_AUX_ENT(AT_ENTRY, e_entry);
 	NEW_AUX_ENT(AT_UID, from_kuid_munged(cred->user_ns, cred->uid));
 	NEW_AUX_ENT(AT_EUID, from_kuid_munged(cred->user_ns, cred->euid));

fs/binfmt_elf_fdpic.c

@@ -506,6 +506,7 @@ static int create_elf_fdpic_tables(struct linux_binprm *bprm,
 	char __user *u_platform, *u_base_platform, *p;
 	int loop;
 	int nr;	/* reset for each csp adjustment */
+	unsigned long flags = 0;
 
 #ifdef CONFIG_MMU
 	/* In some cases (e.g. Hyper-Threading), we want to avoid L1 evictions
@@ -648,7 +649,9 @@ static int create_elf_fdpic_tables(struct linux_binprm *bprm,
 	NEW_AUX_ENT(AT_PHENT,	sizeof(struct elf_phdr));
 	NEW_AUX_ENT(AT_PHNUM,	exec_params->hdr.e_phnum);
 	NEW_AUX_ENT(AT_BASE,	interp_params->elfhdr_addr);
-	NEW_AUX_ENT(AT_FLAGS,	0);
+	if (bprm->interp_flags & BINPRM_FLAGS_PRESERVE_ARGV0)
+		flags |= AT_FLAGS_PRESERVE_ARGV0;
+	NEW_AUX_ENT(AT_FLAGS,	flags);
 	NEW_AUX_ENT(AT_ENTRY,	exec_params->entry_addr);
 	NEW_AUX_ENT(AT_UID,	(elf_addr_t) from_kuid_munged(cred->user_ns, cred->uid));
 	NEW_AUX_ENT(AT_EUID,	(elf_addr_t) from_kuid_munged(cred->user_ns, cred->euid));

fs/binfmt_misc.c

@@ -153,7 +153,9 @@ static int load_misc_binary(struct linux_binprm *bprm)
 	if (bprm->interp_flags & BINPRM_FLAGS_PATH_INACCESSIBLE)
 		goto ret;
 
-	if (!(fmt->flags & MISC_FMT_PRESERVE_ARGV0)) {
+	if (fmt->flags & MISC_FMT_PRESERVE_ARGV0) {
+		bprm->interp_flags |= BINPRM_FLAGS_PRESERVE_ARGV0;
+	} else {
 		retval = remove_arg_zero(bprm);
 		if (retval)
 			goto ret;

include/linux/binfmts.h

@@ -73,6 +73,10 @@ struct linux_binprm {
 #define BINPRM_FLAGS_PATH_INACCESSIBLE_BIT 2
 #define BINPRM_FLAGS_PATH_INACCESSIBLE (1 << BINPRM_FLAGS_PATH_INACCESSIBLE_BIT)
 
+/* preserve argv0 for the interpreter  */
+#define BINPRM_FLAGS_PRESERVE_ARGV0_BIT 3
+#define BINPRM_FLAGS_PRESERVE_ARGV0 (1 << BINPRM_FLAGS_PRESERVE_ARGV0_BIT)
+
 /* Function parameter for binfmt->coredump */
 struct coredump_params {
 	const kernel_siginfo_t *siginfo;

include/uapi/linux/binfmts.h

@@ -18,4 +18,8 @@ struct pt_regs;
 /* sizeof(linux_binprm->buf) */
 #define BINPRM_BUF_SIZE 256
 
+/* preserve argv0 for the interpreter  */
+#define AT_FLAGS_PRESERVE_ARGV0_BIT 0
+#define AT_FLAGS_PRESERVE_ARGV0 (1 << AT_FLAGS_PRESERVE_ARGV0_BIT)
+
 #endif /* _UAPI_LINUX_BINFMTS_H */