Re: [PATCH v3] binfmt_misc: pass binfmt_misc flags to the interpreter

Helge Deller <deller@xxxxxx> · Sat, 13 Feb 2021 08:40:31 +0100

On 6/5/20 6:20 PM, Laurent Vivier wrote:
Le 28/01/2020 à 14:25, Laurent Vivier a écrit :
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.

Signed-off-by: Laurent Vivier <laurent@xxxxxxxxx>

Acked-by: Helge Deller <deller@xxxxxx>

If nobody objects, I'd like to take this patch through the
parisc arch git tree.

It fixes a real-world problem with qemu-user which fails to
preserve the argv[0] argument when the callee of an exec is a
qemu-user target.
This problem leads to build errors on multiple Debian buildd servers
which are using qemu-user as emulation for the target machines.

For details see Debian bug:
http://bugs.debian.org/970460


Helge


---

Notes:
     This can be tested with QEMU from my branch:

       https://github.com/vivier/qemu/commits/binfmt-argv0

     With something like:

       # cp ..../qemu-ppc /chroot/powerpc/jessie

       # qemu-binfmt-conf.sh --qemu-path / --systemd ppc --credential yes \
                             --persistent no --preserve-argv0 yes
       # systemctl restart systemd-binfmt.service
       # cat /proc/sys/fs/binfmt_misc/qemu-ppc
       enabled
       interpreter //qemu-ppc
       flags: POC
       offset 0
       magic 7f454c4601020100000000000000000000020014
       mask ffffffffffffff00fffffffffffffffffffeffff
       # chroot /chroot/powerpc/jessie  sh -c 'echo $0'
       sh

       # qemu-binfmt-conf.sh --qemu-path / --systemd ppc --credential yes \
                             --persistent no --preserve-argv0 no
       # systemctl restart systemd-binfmt.service
       # cat /proc/sys/fs/binfmt_misc/qemu-ppc
       enabled
       interpreter //qemu-ppc
       flags: OC
       offset 0
       magic 7f454c4601020100000000000000000000020014
       mask ffffffffffffff00fffffffffffffffffffeffff
       # chroot /chroot/powerpc/jessie  sh -c 'echo $0'
       /bin/sh

     v3: mix my patch with one from YunQiang Su and my comments on it
         introduce a new flag in the uabi for the AT_FLAGS
     v2: only pass special flags (remove Magic and Enabled flags)

  fs/binfmt_elf.c              | 5 ++++-
  fs/binfmt_elf_fdpic.c        | 5 ++++-
  fs/binfmt_misc.c             | 4 +++-
  include/linux/binfmts.h      | 4 ++++
  include/uapi/linux/binfmts.h | 4 ++++
  5 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index ecd8d2698515..ff918042ceed 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -176,6 +176,7 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
  	unsigned char k_rand_bytes[16];
  	int items;
  	elf_addr_t *elf_info;
+	elf_addr_t flags = 0;
  	int ei_index = 0;
  	const struct cred *cred = current_cred();
  	struct vm_area_struct *vma;
@@ -250,7 +251,9 @@ create_elf_tables(struct linux_binprm *bprm, 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, exec->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));
diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c
index 240f66663543..abb90d82aa58 100644
--- a/fs/binfmt_elf_fdpic.c
+++ b/fs/binfmt_elf_fdpic.c
@@ -507,6 +507,7 @@ 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
@@ -647,7 +648,9 @@ 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));
diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index cdb45829354d..b9acdd26a654 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -154,7 +154,9 @@ 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;
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index b40fc633f3be..265b80d5fd6f 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -78,6 +78,10 @@ struct linux_binprm {
  #define BINPRM_FLAGS_PATH_INACCESSIBLE_BIT 2
  #define BINPRM_FLAGS_PATH_INACCESSIBLE (1 << BINPRM_FLAGS_PATH_INACCESSIBLE_BIT)

+/* if preserve the 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;
diff --git a/include/uapi/linux/binfmts.h b/include/uapi/linux/binfmts.h
index 689025d9c185..a70747416130 100644
--- a/include/uapi/linux/binfmts.h
+++ b/include/uapi/linux/binfmts.h
@@ -18,4 +18,8 @@ struct pt_regs;
  /* sizeof(linux_binprm->buf) */
  #define BINPRM_BUF_SIZE 256

+/* if preserve the 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 */










