[PATCH] linux_nat_target::xfer_partial: Fallback to ptrace

[Keith Seitz <keiths@redhat.com>]

Commit 05c06f318fd9a112529dfc313e6512b399a645e4 enabled GDB
to access memory while threads are running. It did this by accessing
/proc/PID/task/LWP/mem.

Unfortunatley, this interface is not implemented for writing in older kernels
(such as RHEL6). This means that GDB is unable to insert breakpoints on
these hosts:

$ ./gdb -q gdb -ex start
Reading symbols from gdb...
Temporary breakpoint 1 at 0x40fdd5: file ../../src/gdb/gdb.c, line 28.
Starting program: /home/rhel6/fsf/linux/gdb/gdb
Warning:
Cannot insert breakpoint 1.
Cannot access memory at address 0x40fdd5

(gdb)

Before this patch, linux_proc_xfer_memory_partial (previously called
linux_proc_xfer_partial) would return TARGET_XFER_EOF if the write
to /proc/PID/mem failed. [More specifically, linux_proc_xfer_partial would
not "bother for one word," but the effect is the essentially same.]

This status was checked by linux_nat_target::xfer_partial, which would then
fallback to using ptrace to perform the operation.

This is the specific hunk that removed the fallback:

-  xfer = linux_proc_xfer_partial (object, annex, readbuf, writebuf,
-                                 offset, len, xfered_len);
-  if (xfer != TARGET_XFER_EOF)
-    return xfer;
+      return linux_proc_xfer_memory_partial (readbuf, writebuf,
+                                            offset, len, xfered_len);
+    }

   return inf_ptrace_target::xfer_partial (object, annex, readbuf, writebuf,
                                          offset, len, xfered_len);

This patch restores this fallback mechanism, enabling GDB to insert
breakpoints on these older kernels.

Tested on {unix,native-gdbserver,native-extended-gdbserver}/-m{32,64}
on x86_64, s390x, aarch64, and ppc64le.
---
 gdb/linux-nat.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index 740cc0ddfc0..f5490c27bf5 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -3706,8 +3706,12 @@ linux_nat_target::xfer_partial (enum target_object object,
       if (addr_bit < (sizeof (ULONGEST) * HOST_CHAR_BIT))
 	offset &= ((ULONGEST) 1 << addr_bit) - 1;
 
-      return linux_proc_xfer_memory_partial (readbuf, writebuf,
-					     offset, len, xfered_len);
+      enum target_xfer_status xfer
+	= linux_proc_xfer_memory_partial (readbuf, writebuf,
+					  offset, len, xfered_len);
+      if (xfer != TARGET_XFER_EOF)
+	return xfer;
+      /* Fallthrough to ptrace.  */
     }
 
   return inf_ptrace_target::xfer_partial (object, annex, readbuf, writebuf,
-- 
2.36.1