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[PATCH, HACK]: linux-user: handle binfmt-misc P flag as a separate exe name 

Updated: Wed, 31 Aug 2022 12:30:17 +0300

A hackish way to distinguish the case when qemu-user binary is executed
using in-kernel binfmt-misc subsystem with P flag (preserve argv).
We register binfmt interpreter under name /usr/libexec/qemu-binfmt/qemu-foo-binfmt-P
(which is just a symlink to ../../bin/qemu-foo), and if run like that,
qemu-user binary will "know" it should interpret argv[1] & argv[2]
in a special way.

Gbp-Pq: Name linux-user-binfmt-P.diff
Signed-off-by: Cong Liu <liucong2@kylinos.cn>
This commit is contained in:
Michael Tokarev 2021-02-13 13:57:52 +03:00 committed by Cong Liu
parent ad73066ff7
commit 59164a8de4
1 changed files with 28 additions and 3 deletions
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31
linux-user/main.c
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@ -562,7 +562,7 @@ static void usage(int exitcode)
exit(exitcode);
}
static int parse_args(int argc, char **argv)
static int parse_args(int argc, char **argv, bool *preserve_argv0)
{
const char *r;
int optind;
@ -579,6 +579,28 @@ static int parse_args(int argc, char **argv)
}
}
/* HACK alert.
* when run as an interpreter using kernel's binfmt-misc mechanism,
* we have to know where are we (our own binary), where's the binary being run,
* and what it's argv[0] element.
* Only with the P interpreter flag kernel passes all 3 elements as first 3 argv[],
* but we can't distinguish if we were run with or without this P flag.
* So we register a special name with binfmt-misc system, a name which ends up
* in "-binfmt-P", and if our argv[0] ends up with that, we assume we were run
* from kernel's binfmt with P flag and our first 3 args are from kernel.
*/
if (strlen(argv[0]) > sizeof("binfmt-P") &&
strcmp(argv[0] + strlen(argv[0]) - sizeof("binfmt-P"), "-binfmt-P") == 0) {
if (argc < 3) {
(void) fprintf(stderr, "qemu: %s has to be run using kernel binfmt-misc subsystem\n", argv[0]);
exit(EXIT_FAILURE);
}
exec_path = argv[1];
handle_arg_argv0(argv[2]);
*preserve_argv0 = true;
return 2;
}
optind = 1;
for (;;) {
if (optind >= argc) {
@ -648,7 +670,7 @@ int main(int argc, char **argv, char **envp)
int ret;
int execfd;
unsigned long max_reserved_va;
bool preserve_argv0;
bool preserve_argv0 = 0;
error_init(argv[0]);
module_call_init(MODULE_INIT_TRACE);
@ -678,7 +700,7 @@ int main(int argc, char **argv, char **envp)
qemu_add_opts(&qemu_trace_opts);
qemu_plugin_add_opts();
optind = parse_args(argc, argv);
optind = parse_args(argc, argv, &preserve_argv0);
qemu_set_log_filename_flags(last_log_filename,
last_log_mask | (enable_strace * LOG_STRACE),
@ -717,7 +739,9 @@ int main(int argc, char **argv, char **envp)
/*
* get binfmt_misc flags
* but only if not already done by parse_args() above
*/
if (!preserve_argv0) {
preserve_argv0 = !!(qemu_getauxval(AT_FLAGS) & AT_FLAGS_PRESERVE_ARGV0);
/*
@ -728,6 +752,7 @@ int main(int argc, char **argv, char **envp)
if (optind + 1 < argc && preserve_argv0) {
optind++;
}
}
if (cpu_model == NULL) {
cpu_model = cpu_get_model(get_elf_eflags(execfd));