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Re: [RFA] Problem after hitting breakpoint on Windows (with GDBserver)


Hi Pedro,

Thanks for the review!

> > +  if (val == 0)
> > +    memcpy (bp_tgt->shadow_contents, readbuf, bp_tgt->placed_size);
> >  
> >    /* Write the breakpoint.  */
> >    if (val == 0)
> 
> Merge?

I actually started that way, with the two blocks merged. But I felt
that it was breaking the separation between the two steps. With the
comments clearly separating the two steps, I didn't want to break
that unless asked. So now I changed it.

> > +   As a limitation, MYADDR must not be the shadow_contents buffer of one
> 
> I wouldn't call it a limitation; it's more a design choice thing, like
> memcpy doesn't handle overlapping buffers.

OK - I just removed the "As a limitation" from the comments.

> Otherwise this is fine with me.

Thanks! Attached is a new version of the patch. The only changes
should be the changes you pointed out.

> An assertion in breakpoint_xfer_memory to catch that READBUF or
> WRITEBUF doesn't overlap bp->target_info.shadow_contents would be
> nice.

I thought about that, but decided to look at that separately, since
it doesn't help correctness, and can potentially be a little expensive
(at least compared to just allocating a buffer on the heap - I think!).

But I don't mind writing a patch - probably a function in breakpoint.c
and a gdb_assert calling that breakpoint?

> As we discussed yesterday on IRC, the current code always reinserts
> locations, which means that gdb > 7.4 now does an extra read off of
> inferior memory to fill the shadow on breakpoint re-sets.  That'd be
> possible to avoid (and avoiding this whole problem along the way),
> though I think Joel's patch is a good one even knowing that.

Agreed. We've seen a number of emails on the GDB lists showing that
every increase in traffic can have a significant impact in some
situations...

-- 
Joel
>From 6f9ed095afee3065abf3dcda8e364153734938f4 Mon Sep 17 00:00:00 2001
From: Joel Brobecker <brobecker@adacore.com>
Date: Mon, 12 Mar 2012 22:44:05 +0100
Subject: [PATCH] Problem after hitting breakpoint on Windows (with GDBserver)

gdb/ChangeLog:

        * breakpoint.c (insert_bp_location): Do not wipe bl->target_info out.

        * target.h (target_read): Document limitation.
        * target.c (memory_xfer_partial, target_xfer_partial)
        (target_read_memory): Document limitation.
        * mem-break.c: #include "gdb_string.h".
        (default_memory_insert_breakpoint): Do not call target_read_memory
        with a pointer to the breakpoint's shadow_contents buffer.  Use
        a local buffer instead.
        * m32r-tdep.c (m32r_memory_insert_breakpoint): Ditto.
---
 gdb/breakpoint.c |   11 +++++++++--
 gdb/m32r-tdep.c  |    3 ++-
 gdb/mem-break.c  |   17 +++++++++++------
 gdb/target.c     |   19 ++++++++++++++++---
 gdb/target.h     |    7 ++++++-
 5 files changed, 44 insertions(+), 13 deletions(-)

diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index d35704d..95d8783 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -2082,8 +2082,15 @@ insert_bp_location (struct bp_location *bl,
   if (!should_be_inserted (bl) || (bl->inserted && !bl->needs_update))
     return 0;
 
-  /* Initialize the target-specific information.  */
-  memset (&bl->target_info, 0, sizeof (bl->target_info));
+  /* Note we don't initialize bl->target_info, as that wipes out
+     the breakpoint location's shadow_contents if the breakpoint
+     is still inserted at that location.  This in turn breaks
+     target_read_memory which depends on these buffers when
+     a memory read is requested at the breakpoint location:
+     Once the target_info has been wiped, we fail to see that
+     we have a breakpoint inserted at that address and thus
+     read the breakpoint instead of returning the data saved in
+     the breakpoint location's shadow contents.  */
   bl->target_info.placed_address = bl->address;
   bl->target_info.placed_address_space = bl->pspace->aspace;
   bl->target_info.length = bl->length;
diff --git a/gdb/m32r-tdep.c b/gdb/m32r-tdep.c
index 72872bd..d504eb3 100644
--- a/gdb/m32r-tdep.c
+++ b/gdb/m32r-tdep.c
@@ -85,7 +85,7 @@ m32r_memory_insert_breakpoint (struct gdbarch *gdbarch,
   CORE_ADDR addr = bp_tgt->placed_address;
   int val;
   gdb_byte buf[4];
-  gdb_byte *contents_cache = bp_tgt->shadow_contents;
+  gdb_byte contents_cache[4];
   gdb_byte bp_entry[] = { 0x10, 0xf1 };	/* dpt */
 
   /* Save the memory contents.  */
@@ -93,6 +93,7 @@ m32r_memory_insert_breakpoint (struct gdbarch *gdbarch,
   if (val != 0)
     return val;			/* return error */
 
+  memcpy (bp_tgt->shadow_contents, contents_cache, 4);
   bp_tgt->placed_size = bp_tgt->shadow_len = 4;
 
   /* Determine appropriate breakpoint contents and size for this address.  */
diff --git a/gdb/mem-break.c b/gdb/mem-break.c
index 7d0e3f1..bd34fb2 100644
--- a/gdb/mem-break.c
+++ b/gdb/mem-break.c
@@ -29,6 +29,7 @@
 #include "breakpoint.h"
 #include "inferior.h"
 #include "target.h"
+#include "gdb_string.h"
 
 
 /* Insert a breakpoint on targets that don't have any better
@@ -46,6 +47,7 @@ default_memory_insert_breakpoint (struct gdbarch *gdbarch,
 {
   int val;
   const unsigned char *bp;
+  gdb_byte *readbuf;
 
   /* Determine appropriate breakpoint contents and size for this address.  */
   bp = gdbarch_breakpoint_from_pc
@@ -53,15 +55,18 @@ default_memory_insert_breakpoint (struct gdbarch *gdbarch,
   if (bp == NULL)
     error (_("Software breakpoints not implemented for this target."));
 
-  /* Save the memory contents.  */
+  /* Save the memory contents in the shadow_contents buffer and then
+     write the breakpoint instruction.  */
   bp_tgt->shadow_len = bp_tgt->placed_size;
-  val = target_read_memory (bp_tgt->placed_address, bp_tgt->shadow_contents,
+  readbuf = alloca (bp_tgt->placed_size);
+  val = target_read_memory (bp_tgt->placed_address, readbuf,
 			    bp_tgt->placed_size);
-
-  /* Write the breakpoint.  */
   if (val == 0)
-    val = target_write_raw_memory (bp_tgt->placed_address, bp,
-				   bp_tgt->placed_size);
+    {
+      memcpy (bp_tgt->shadow_contents, readbuf, bp_tgt->placed_size);
+      val = target_write_raw_memory (bp_tgt->placed_address, bp,
+				     bp_tgt->placed_size);
+    }
 
   return val;
 }
diff --git a/gdb/target.c b/gdb/target.c
index cffea2c..2afae74 100644
--- a/gdb/target.c
+++ b/gdb/target.c
@@ -1608,7 +1608,11 @@ memory_xfer_partial_1 (struct target_ops *ops, enum target_object object,
 }
 
 /* Perform a partial memory transfer.  For docs see target.h,
-   to_xfer_partial.  */
+   to_xfer_partial.
+
+   In addition, READBUF must not be the shadow_contents buffer of
+   one of the breakpoint locations.  Otherwise, this shadow_contents
+   buffer will become corrupted.  */
 
 static LONGEST
 memory_xfer_partial (struct target_ops *ops, enum target_object object,
@@ -1665,7 +1669,12 @@ make_show_memory_breakpoints_cleanup (int show)
 		       (void *) (uintptr_t) current);
 }
 
-/* For docs see target.h, to_xfer_partial.  */
+/* For docs see target.h, to_xfer_partial.
+
+   In addition, READBUF must not be the shadow_contents buffer of
+   one of the breakpoint locations when OBJECT is TARGET_OBJECT_MEMORY
+   or TARGET_OBJECT_STACK_MEMORY.  Otherwise, this shadow_contents
+   buffer will become corrupted.  */
 
 static LONGEST
 target_xfer_partial (struct target_ops *ops,
@@ -1754,7 +1763,11 @@ target_xfer_partial (struct target_ops *ops,
    filling the buffer with good data.  There is no way for the caller to know
    how much good data might have been transfered anyway.  Callers that can
    deal with partial reads should call target_read (which will retry until
-   it makes no progress, and then return how much was transferred).  */
+   it makes no progress, and then return how much was transferred).
+
+   MYADDR must not be the shadow_contents buffer of one of the breakpoint
+   locations.  Passing a breakpoint's shadow_contents buffer will cause
+   that buffer to become corrupted.  */
 
 int
 target_read_memory (CORE_ADDR memaddr, gdb_byte *myaddr, int len)
diff --git a/gdb/target.h b/gdb/target.h
index 50a0ea6..69d7a5d 100644
--- a/gdb/target.h
+++ b/gdb/target.h
@@ -310,7 +310,12 @@ DEF_VEC_P(static_tracepoint_marker_p);
    transfer is not supported or otherwise fails.  Return of a positive
    value less than LEN indicates that no further transfer is possible.
    Unlike the raw to_xfer_partial interface, callers of these
-   functions do not need to retry partial transfers.  */
+   functions do not need to retry partial transfers.
+
+   When OBJECT is TARGET_OBJECT_MEMORY or TARGET_OBJECT_STACK_MEMORY,
+   MYADDR must not be the shadow_contents buffer of one of the breakpoint
+   locations.  Passing a breakpoint's shadow_contents buffer in that
+   situation will cause that buffer to become corrupted.  */
 
 extern LONGEST target_read (struct target_ops *ops,
 			    enum target_object object,
-- 
1.7.1


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