Gracefully handle the case of missing libvsdo.so

Current implementation of application::prepare_argv() aborts execution
if it fails to load libvdso library. So far libvdso library is only
used by Golang programs and other applications can function just fine
without it. Unfortunately this prevents older OSv images (especially
those created by Mikelangelo capstan) to boot.

This patch changes prepare_argv() implementation to print warning
if libvdso is missing and skip passing its address in auxv array.

Fixes #966

Signed-off-by: Waldemar Kozaczuk <jwkozaczuk@gmail.com>

core/app.cc

@@ -6,6 +6,7 @@
  */
 
 #include <osv/app.hh>
+#include <osv/stubbing.hh>
 #include <string>
 #include <osv/run.hh>
 #include <osv/power.hh>
@@ -359,8 +360,16 @@ void application::prepare_argv(elf::program *program)
         envcount++;
     }
 
+    // Load vdso library if available
+    int auxv_parameters_count = 2;
+    _libvdso = program->get_library("libvdso.so");
+    if (!_libvdso) {
+        auxv_parameters_count--;
+        WARN_ONCE("application::prepare_argv(): missing libvdso.so -> may prevent shared libraries specifically Golang ones from functioning\n");
+    }
+
     // Allocate the continuous buffer for argv[] and envp[]
-    _argv.reset(new char*[_args.size() + 1 + envcount + 1 + sizeof(Elf64_auxv_t) * 3]);
+    _argv.reset(new char*[_args.size() + 1 + envcount + 1 + sizeof(Elf64_auxv_t) * (auxv_parameters_count + 1)]);
 
     // Fill the argv part of these buffers
     char *ab = _argv_buf.get();
@@ -380,22 +389,21 @@ void application::prepare_argv(elf::program *program)
     }
     contig_argv[_args.size() + 1 + envcount] = nullptr;
 
-    _libvdso = program->get_library("libvdso.so");
-    if (!_libvdso) {
-        abort("could not load libvdso.so\n");
-    }
 
     // Pass the VDSO library to the application.
     Elf64_auxv_t* _auxv =
         reinterpret_cast<Elf64_auxv_t *>(&contig_argv[_args.size() + 1 + envcount + 1]);
-    _auxv[0].a_type = AT_SYSINFO_EHDR;
-    _auxv[0].a_un.a_val = reinterpret_cast<uint64_t>(_libvdso->base());
+    int auxv_idx = 0;
+    if (_libvdso) {
+        _auxv[auxv_idx].a_type = AT_SYSINFO_EHDR;
+        _auxv[auxv_idx++].a_un.a_val = reinterpret_cast<uint64_t>(_libvdso->base());
+    }
 
-    _auxv[1].a_type = AT_PAGESZ;
-    _auxv[1].a_un.a_val = sysconf(_SC_PAGESIZE);
+    _auxv[auxv_idx].a_type = AT_PAGESZ;
+    _auxv[auxv_idx++].a_un.a_val = sysconf(_SC_PAGESIZE);
 
-    _auxv[2].a_type = AT_NULL;
-    _auxv[2].a_un.a_val = 0;
+    _auxv[auxv_idx].a_type = AT_NULL;
+    _auxv[auxv_idx].a_un.a_val = 0;
 }
 
 void application::run_main()