92 lines
3.5 KiB
Diff
92 lines
3.5 KiB
Diff
Subject: [PATCH, HACK]: linux-user: handle binfmt-misc P flag as a separate exe name
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From: Michael Tokarev <mjt@tls.msk.ru>
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Date: Sat, 13 Feb 2021 13:57:52 +0300
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Updated: Wed, 31 Aug 2022 12:30:17 +0300
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Forwarded: https://patchwork.ozlabs.org/project/qemu-devel/patch/27dfe8eb-adce-8db4-f28b-c42858b086db@msgid.tls.msk.ru/
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Upstream-Status: rejected this simple userspace solution in favour of more complex in-kernel one
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A hackish way to distinguish the case when qemu-user binary is executed
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using in-kernel binfmt-misc subsystem with P flag (preserve argv).
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We register binfmt interpreter under name /usr/libexec/qemu-binfmt/qemu-foo-binfmt-P
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(which is just a symlink to ../../bin/qemu-foo), and if run like that,
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qemu-user binary will "know" it should interpret argv[1] & argv[2]
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in a special way.
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diff --git a/linux-user/main.c b/linux-user/main.c
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index 9277df2e9d..bd323f70bc 100644
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--- a/linux-user/main.c
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+++ b/linux-user/main.c
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@@ -600,7 +600,7 @@ static void usage(int exitcode)
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exit(exitcode);
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}
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-static int parse_args(int argc, char **argv)
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+static int parse_args(int argc, char **argv, bool *preserve_argv0)
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{
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const char *r;
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int optind;
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@@ -617,6 +617,28 @@ static int parse_args(int argc, char **argv)
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}
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}
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+ /* HACK alert.
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+ * when run as an interpreter using kernel's binfmt-misc mechanism,
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+ * we have to know where are we (our own binary), where's the binary being run,
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+ * and what it's argv[0] element.
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+ * Only with the P interpreter flag kernel passes all 3 elements as first 3 argv[],
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+ * but we can't distinguish if we were run with or without this P flag.
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+ * So we register a special name with binfmt-misc system, a name which ends up
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+ * in "-binfmt-P", and if our argv[0] ends up with that, we assume we were run
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+ * from kernel's binfmt with P flag and our first 3 args are from kernel.
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+ */
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+ if (strlen(argv[0]) > sizeof("binfmt-P") &&
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+ strcmp(argv[0] + strlen(argv[0]) - sizeof("binfmt-P"), "-binfmt-P") == 0) {
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+ if (argc < 3) {
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+ (void) fprintf(stderr, "qemu: %s has to be run using kernel binfmt-misc subsystem\n", argv[0]);
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+ exit(EXIT_FAILURE);
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+ }
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+ exec_path = argv[1];
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+ handle_arg_argv0(argv[2]);
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+ *preserve_argv0 = true;
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+ return 2;
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+ }
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+
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optind = 1;
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for (;;) {
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if (optind >= argc) {
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@@ -687,7 +709,7 @@ int main(int argc, char **argv, char **envp)
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int execfd;
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int host_page_size;
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unsigned long max_reserved_va;
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- bool preserve_argv0;
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+ bool preserve_argv0 = 0;
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error_init(argv[0]);
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module_call_init(MODULE_INIT_TRACE);
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@@ -726,7 +748,7 @@ int main(int argc, char **argv, char **envp)
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qemu_add_opts(&qemu_trace_opts);
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qemu_plugin_add_opts();
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- optind = parse_args(argc, argv);
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+ optind = parse_args(argc, argv, &preserve_argv0);
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qemu_set_log_filename_flags(last_log_filename,
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last_log_mask | (enable_strace * LOG_STRACE),
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@@ -770,7 +792,9 @@ int main(int argc, char **argv, char **envp)
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/*
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* get binfmt_misc flags
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+ * but only if not already done by parse_args() above
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*/
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+ if (!preserve_argv0) {
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preserve_argv0 = !!(qemu_getauxval(AT_FLAGS) & AT_FLAGS_PRESERVE_ARGV0);
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/*
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@@ -781,6 +805,7 @@ int main(int argc, char **argv, char **envp)
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if (optind + 1 < argc && preserve_argv0) {
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optind++;
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}
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+ }
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if (cpu_model == NULL) {
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cpu_model = cpu_get_model(get_elf_eflags(execfd));
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