linux-user: manage binfmt-misc preserve-arg[0] flag

Add --preserve-argv0 in qemu-binfmt-conf.sh to configure the preserve-argv0
flag.

This patch allows to use new flag in AT_FLAGS to detect if
preserve-argv0 is configured for this interpreter:
argv[0] (the full pathname provided by binfmt-misc) is removed and
replaced by argv[1] (the original argv[0] provided by binfmt-misc when
'P'/preserve-arg[0] is set)

For instance with this patch and kernel support for AT_FLAGS:

  $ sudo chroot m68k-chroot sh -c 'echo $0'
  sh

without this patch:

  $ sudo chroot m68k-chroot sh -c 'echo $0'
  /usr/bin/sh

The new flag is available in kernel (v5.12) since:
2347961b11d4 ("binfmt_misc: pass binfmt_misc flags to the interpreter")

This can be tested 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

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210222105004.1642234-1-laurent@vivier.eu>

1 files changed, 24 insertions, 0 deletions

diff --git a/linux-user/main.c b/linux-user/main.c
index 4f4746dce8..f956afccab 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -26,6 +26,7 @@
 #include <sys/syscall.h>
 #include <sys/resource.h>
 #include <sys/shm.h>
+#include <linux/binfmts.h>
 
 #include "qapi/error.h"
 #include "qemu.h"
@@ -49,6 +50,11 @@
 #include "cpu_loop-common.h"
 #include "crypto/init.h"
 
+#ifndef AT_FLAGS_PRESERVE_ARGV0
+#define AT_FLAGS_PRESERVE_ARGV0_BIT 0
+#define AT_FLAGS_PRESERVE_ARGV0 (1 << AT_FLAGS_PRESERVE_ARGV0_BIT)
+#endif
+
 char *exec_path;
 
 int singlestep;
@@ -632,6 +638,7 @@ int main(int argc, char **argv, char **envp)
     int execfd;
     int log_mask;
     unsigned long max_reserved_va;
+    bool preserve_argv0;
 
     error_init(argv[0]);
     module_call_init(MODULE_INIT_TRACE);
@@ -688,6 +695,9 @@ int main(int argc, char **argv, char **envp)
 
     init_qemu_uname_release();
 
+    /*
+     * Manage binfmt-misc open-binary flag
+     */
     execfd = qemu_getauxval(AT_EXECFD);
     if (execfd == 0) {
         execfd = open(exec_path, O_RDONLY);
@@ -697,6 +707,20 @@ int main(int argc, char **argv, char **envp)
         }
     }
 
+    /*
+     * get binfmt_misc flags
+     */
+    preserve_argv0 = !!(qemu_getauxval(AT_FLAGS) & AT_FLAGS_PRESERVE_ARGV0);
+
+    /*
+     * Manage binfmt-misc preserve-arg[0] flag
+     *    argv[optind]     full path to the binary
+     *    argv[optind + 1] original argv[0]
+     */
+    if (optind + 1 < argc && preserve_argv0) {
+        optind++;
+    }
+
     if (cpu_model == NULL) {
         cpu_model = cpu_get_model(get_elf_eflags(execfd));
     }