Actually better error message for mount() and ENOENT

The previous change assumed that we could mount over a dangling symlink, which is true for the `mount` command-line tool, but not actually for the `mount()` syscall.
Also removed the odd `%s%s%s%s` string format in the message in favor of an easier to read if-else.

PiperOrigin-RevId: 676366941
Change-Id: Ib0bab7e18adadbb4dda79bbaa404f44bc6141cdb

sandboxed_api/sandbox2/mounts.cc

@@ -641,19 +641,21 @@ void MountWithDefaults(const std::string& source, const std::string& target,
  // File does not exist (anymore). This may be the case when trying to
  // gather stack-traces on SAPI crashes. The sandboxee application is a
  // memfd file that is not existing anymore.
- // Check which file/dir of the call is actually missing. Do not follow
- // symlinks here as it's valid to "mount over" a dangling symlink.
+ // Check which file/dir of the call is actually missing.
  bool have_source =
- file_util::fileops::Exists(source, /*fully_resolve=*/false);
+ file_util::fileops::Exists(source, /*fully_resolve=*/true);
  bool have_target =
- file_util::fileops::Exists(target, /*fully_resolve=*/false);
- SAPI_RAW_LOG(WARNING,
- "Could not mount %s (source) to %s (target): %s%s%s%s exist",
- source.c_str(), target.c_str(),
- have_source || have_target ? "" : "neither ",
- have_source ? "" : "source",
- have_source || have_target ? "" : " nor ",
- have_target ? "" : "target");
+ file_util::fileops::Exists(target, /*fully_resolve=*/true);
+ const char* detail = "unknown error, source and target exist";
+ if (!have_source && !have_target) {
+ detail = "neither source nor target exist";
+ } else if (!have_source) {
+ detail = "source does not exist";
+ } else if (!have_target) {
+ detail = "target does not exist";
+ }
+ SAPI_RAW_LOG(WARNING, "Could not mount %s (source) to %s (target): %s",
+ source.c_str(), target.c_str(), detail);
  return;
  }
  SAPI_RAW_PLOG(FATAL, "mounting %s to %s failed (flags=%s)", source, target,