gdbserver/linux: Access memory even if threads are running

Similarly to how the native Linux target was changed
and subsequently reworked in these commits:

 05c06f318fd9 Linux: Access memory even if threads are running
 8a89ddbda2ec Avoid /proc/pid/mem races (PR 28065)

... teach GDBserver to access memory even when the current thread is
running, by always accessing memory via /proc/PID/mem.

The existing comment:

  /* Neither ptrace nor /proc/PID/mem allow accessing memory through a
     running LWP.  */

... is incorrect for /proc/PID/mem does allow that.

Actually, from GDB's perspective, GDBserver could already access
memory while threads were running, but at the expense of pausing all
threads for the duration of the memory access, via
prepare_to_access_memory.  This new implementation does not require
pausing any thread, thus
linux_process_target::prepare_to_access_memory /
linux_process_target::done_accessing_memory become nops.  A subsequent
patch will remove the whole prepare_to_access_memory infrastructure
completely.

The GDBserver linux-low.cc implementation is simpler than GDB's
linux-nat.c's, because GDBserver always adds the unfollowed vfork/fork
children to the process list immediately when the fork/vfork event is
seen out of ptrace.  I.e., there's no need to keep the file descriptor
stored on a side map, we can store it directly in the process
structure.

Change-Id: I0abfd782ceaa4ddce8d3e5f3e2dfc5928862ef61

1 files changed, 7 insertions, 4 deletions

diff --git a/gdbserver/linux-low.h b/gdbserver/linux-low.h
index 27cc964..4284fb3 100644
--- a/gdbserver/linux-low.h
+++ b/gdbserver/linux-low.h
@@ -127,6 +127,9 @@ struct process_info_private
 
   /* &_r_debug.  0 if not yet determined.  -1 if no PT_DYNAMIC in Phdrs.  */
   CORE_ADDR r_debug;
+
+  /* The /proc/pid/mem file used for reading/writing memory.  */
+  int mem_fd;
 };
 
 struct lwp_info;
@@ -163,10 +166,6 @@ public:
 
   void store_registers (regcache *regcache, int regno) override;
 
-  int prepare_to_access_memory () override;
-
-  void done_accessing_memory () override;
-
   int read_memory (CORE_ADDR memaddr, unsigned char *myaddr,
 		   int len) override;
 
@@ -544,6 +543,10 @@ private:
      data.  */
   process_info *add_linux_process (int pid, int attached);
 
+  /* Same as add_linux_process, but don't open the /proc/PID/mem file
+     yet.  */
+  process_info *add_linux_process_no_mem_file (int pid, int attached);
+
   /* Add a new thread.  */
   lwp_info *add_lwp (ptid_t ptid);