Add support for symbol table in .com files

This change fixes minor bugs and adds a feature, which lets us store the
ELF symbol table, inside the ZIP directory. We use the path /zip/.symtab
which can be safely removed using a zip editing tool, to make the binary
smaller after compilation. This supplements the existing method of using
a separate .com.dbg file, which is still supported. The intent is people
don't always know that it's a good idea to download the debug file. It's
not great having someone's first experience be a crash report, that only
has numbers rather than symbols. This will help fix that!

libc/runtime/getsymboltable.c

@@ -0,0 +1,133 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2020 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/assert.h"
+#include "libc/bits/bits.h"
+#include "libc/bits/weaken.h"
+#include "libc/calls/strace.internal.h"
+#include "libc/intrin/kprintf.h"
+#include "libc/macros.internal.h"
+#include "libc/runtime/runtime.h"
+#include "libc/runtime/symbols.internal.h"
+#include "libc/str/str.h"
+#include "libc/str/undeflate.h"
+#include "libc/x/x.h"
+#include "libc/zip.h"
+#include "libc/zipos/zipos.internal.h"
+
+/**
+ * Looks for `.symtab` in zip central directory.
+ */
+noasan static ssize_t FindSymtabInZip(struct Zipos *zipos) {
+  size_t i, n, c;
+  c = GetZipCdirOffset(zipos->cdir);
+  n = GetZipCdirRecords(zipos->cdir);
+  for (i = 0; i < n; ++i, c += ZIP_CFILE_HDRSIZE(zipos->map + c)) {
+    if (ZIP_CFILE_NAMESIZE(zipos->map + c) == 7 &&
+        READ32LE(ZIP_CFILE_NAME(zipos->map + c + 0)) == READ32LE(".sym") &&
+        READ16LE(ZIP_CFILE_NAME(zipos->map + c + 4)) == READ16LE("ta") &&
+        *ZIP_CFILE_NAME(zipos->map + c + 6) == 'b') {
+      return c;
+    }
+  }
+  return -1;
+}
+
+/**
+ * Reads symbol table from zip directory.
+ * @note This code can't depend on dlmalloc()
+ */
+noasan static struct SymbolTable *GetSymbolTableFromZip(struct Zipos *zipos) {
+  ssize_t cf, lf;
+  size_t size, size2;
+  struct DeflateState ds;
+  struct SymbolTable *res = 0;
+  if ((cf = FindSymtabInZip(zipos)) != -1) {
+    lf = GetZipCfileOffset(zipos->map + cf);
+    size = GetZipLfileUncompressedSize(zipos->map + lf);
+    size2 = ROUNDUP(size, FRAMESIZE);
+    if ((res = mapanon(size2))) {
+      switch (ZIP_LFILE_COMPRESSIONMETHOD(zipos->map + lf)) {
+        case kZipCompressionNone:
+          memcpy(res, (void *)ZIP_LFILE_CONTENT(zipos->map + lf), size);
+          break;
+        case kZipCompressionDeflate:
+          if (undeflate(res, size, (void *)ZIP_LFILE_CONTENT(zipos->map + lf),
+                        GetZipLfileCompressedSize(zipos->map + lf),
+                        &ds) == -1) {
+            munmap(res, size2);
+            res = 0;
+          }
+          break;
+        default:
+          munmap(res, size2);
+          res = 0;
+          break;
+      }
+    }
+  }
+  STRACE("GetSymbolTableFromZip() → %p", res);
+  return res;
+}
+
+/**
+ * Reads symbol table from .com.dbg file.
+ * @note This code can't depend on dlmalloc()
+ */
+noasan static struct SymbolTable *GetSymbolTableFromElf(void) {
+  return OpenSymbolTable(FindDebugBinary());
+}
+
+/**
+ * Returns symbol table singleton.
+ *
+ * This uses multiple strategies to find the symbol table. The first
+ * strategy, depends on whether or not the following is linked:
+ *
+ *     STATIC_YOINK("__zipos_get");
+ *
+ * In that case, the symbol table may be read from `/zip/.symtab` which
+ * is generated by `o//tool/build/symtab.com`. The second strategy is to
+ * look for the concomitant `.com.dbg` executable, which may very well
+ * be the one currently executing, or it could be placed in the same
+ * folder as your `.com` binary, or lastly, it could be explicitly
+ * specified via the `COMDBG` environment variable.
+ *
+ * Function tracing is disabled throughout the duration of this call.
+ * Backtraces and other core runtime functionality depend on this.
+ *
+ * @return symbol table, or NULL w/ errno on first call
+ */
+noasan struct SymbolTable *GetSymbolTable(void) {
+  struct Zipos *z;
+  static struct SymbolTable *t;
+  if (!t) {
+    ++g_ftrace;
+    if (weaken(__zipos_get) && (z = weaken(__zipos_get)())) {
+      if ((t = GetSymbolTableFromZip(z))) {
+        t->names = (uint32_t *)((char *)t + t->names_offset);
+        t->name_base = (char *)((char *)t + t->name_base_offset);
+      }
+    }
+    if (!t) {
+      t = GetSymbolTableFromElf();
+    }
+    --g_ftrace;
+  }
+  return t;
+}