gdb/riscv: delete target descriptions when gdb exits

It was pointed out on IRC that the RISC-V target allocates target
descriptions and stores them in a global map, and doesn't delete these
target descriptions when GDB shuts down.

This isn't a particular problem, the total number of target
descriptions we can create is very limited so creating these on demand
and holding them for the entire run on GDB seems reasonable.

However, not deleting these objects on GDB exit means extra warnings
are printed from tools like valgrind, and the address sanitiser,
making it harder to spot real issues.  As it's reasonably easy to have
GDB correctly delete these objects on exit, lets just do that.

I started by noticing that we already have a target_desc_up type, a
wrapper around unique_ptr that calls a function that will correctly
delete target descriptions, so I want to use that, but....

...that type is declared in gdb/target-descriptions.h.  If I try to
include that file in gdb/arch/riscv.c I run into a problem, that file
is compiled into both GDB and GDBServer.

OK, I could guard the include with #ifdef, but surely we can do
better.

So then I decided to move the target_desc_up type into
gdbsupport/tdesc.h, this is the interface file for generic code shared
between GDB and GDBserver (relating to target descriptions).  The
actual implementation for the delete function still lives in
gdb/target-description.c, but now gdb/arch/riscv.c can see the
declaration.  Problem solved....

... but, though RISC-V doesn't use it I've now exposed the
target_desc_up type to gdbserver, so in future someone _might_ start
using it, which is fine, except right now there's no definition of the
delete function - remember the delete I used is only defined in GDB
code.

No problem, I add an implementation of the delete operator into
gdbserver/tdesc.cc, and all is good..... except....

I start getting this error from GCC:

  tdesc.cc:109:10: error: deleting object of polymorphic class type ‘target_desc’ which has non-virtual destructor might cause undefined behavior [-Werror=delete-non-virtual-dtor]

Which is caused because gdbserver's target_desc type inherits from
tdesc_element which has a virtual method, and so GCC worries that
target_desc might be used as a base class.

The solution is to declare gdbserver's target_desc class as final.
This is fine so long as we never intent to inherit from
target_desc (in gdbserver).  But if we did then we'd want to make
target_desc's destructor virtual anyway, so the error above would be
resolved, and there wouldn't be an issue.

gdb/ChangeLog:

	* arch/riscv.c (riscv_tdesc_cache): Change map type.
	(riscv_lookup_target_description): Return pointer out of
	unique_ptr.
	* target-descriptions.c (allocate_target_description): Add
	comment.
	(target_desc_deleter::operator()): Likewise.
	* target-descriptions.h (struct target_desc_deleter): Moved to
	gdbsupport/tdesc.h.
	(target_desc_up): Likewise.

gdbserver/ChangeLog:

	* tdesc.cc (allocate_target_description): Add header comment.
	(target_desc_deleter::operator()): New function.
	* tdesc.h (struct target_desc): Declare as final.

gdbsupport/ChangeLog:

	* tdesc.h (struct target_desc_deleter): Moved here
	from gdb/target-descriptions.h, extend comment.
	(target_desc_up): Likewise.

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 1b5bc8b458ae11b31189dd4635da168bf8a9ccd7..5a01458a46a44715c9c6622bfee07f2177423471 100644 (file)
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,15 @@
+2020-07-17  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+       * arch/riscv.c (riscv_tdesc_cache): Change map type.
+       (riscv_lookup_target_description): Return pointer out of
+       unique_ptr.
+       * target-descriptions.c (allocate_target_description): Add
+       comment.
+       (target_desc_deleter::operator()): Likewise.
+       * target-descriptions.h (struct target_desc_deleter): Moved to
+       gdbsupport/tdesc.h.
+       (target_desc_up): Likewise.
+
 2020-07-17  Tom Tromey  <tromey@adacore.com>
 
        * linux-nat.c (linux_nat_target::supports_non_stop)

diff --git a/gdb/arch/riscv.c b/gdb/arch/riscv.c
index a02c18b4c916907f7205ee693c4838e4f21776c6..e43aafc1c2fb7e9292aed6894e8f1e58d841dc83 100644 (file)
--- a/gdb/arch/riscv.c
+++ b/gdb/arch/riscv.c
@@ -91,7 +91,7 @@ struct riscv_gdbarch_features_hasher
 /* Cache of previously seen target descriptions, indexed by the feature set
    that created them.  */
 static std::unordered_map<riscv_gdbarch_features,
-                         const target_desc *,
+                         const target_desc_up,
                          riscv_gdbarch_features_hasher> riscv_tdesc_cache;
 
 /* See arch/riscv.h.  */
@@ -99,10 +99,12 @@ static std::unordered_map<riscv_gdbarch_features,
 const target_desc *
 riscv_lookup_target_description (const struct riscv_gdbarch_features features)
 {
-  /* Lookup in the cache.  */
+  /* Lookup in the cache.  If we find it then return the pointer out of
+     the target_desc_up (which is a unique_ptr).  This is safe as the
+     riscv_tdesc_cache will exist until GDB exits.  */
   const auto it = riscv_tdesc_cache.find (features);
   if (it != riscv_tdesc_cache.end ())
-    return it->second;
+    return it->second.get ();
 
   target_desc *tdesc = riscv_create_target_description (features);
 

diff --git a/gdb/target-descriptions.c b/gdb/target-descriptions.c
index 2f4b17727b8d11cd7a4a0ea208391fb74b486d82..20d624c0c6524e76dbd51082ef2f2100eb19e8b6 100644 (file)
--- a/gdb/target-descriptions.c
+++ b/gdb/target-descriptions.c
@@ -1206,12 +1206,16 @@ tdesc_create_feature (struct target_desc *tdesc, const char *name)
   return new_feature;
 }
 
+/* See gdbsupport/tdesc.h.  */
+
 struct target_desc *
 allocate_target_description (void)
 {
   return new target_desc ();
 }
 
+/* See gdbsupport/tdesc.h.  */
+
 void
 target_desc_deleter::operator() (struct target_desc *target_desc) const
 {

diff --git a/gdb/target-descriptions.h b/gdb/target-descriptions.h
index 6d842bf07ed307fe7e89db2ab771f074e7cbf8e6..66a2c213dc2ff4094ca014b8d1d8cce7f976e554 100644 (file)
--- a/gdb/target-descriptions.h
+++ b/gdb/target-descriptions.h
@@ -225,18 +225,6 @@ struct type *tdesc_find_type (struct gdbarch *gdbarch, const char *id);
 int tdesc_register_in_reggroup_p (struct gdbarch *gdbarch, int regno,
                                  struct reggroup *reggroup);
 
-
-/* A deleter adapter for a target desc.  */
-
-struct target_desc_deleter
-{
-  void operator() (struct target_desc *desc) const;
-};
-
-/* A unique pointer specialization that holds a target_desc.  */
-
-typedef std::unique_ptr<target_desc, target_desc_deleter> target_desc_up;
-
 /* Methods for constructing a target description.  */
 
 void set_tdesc_architecture (struct target_desc *,

diff --git a/gdbserver/ChangeLog b/gdbserver/ChangeLog
index 1ee716f52d7eda530b860e503c6623e3799812fc..9d615870d17741326ddd9afdc25352e66343e818 100644 (file)
--- a/gdbserver/ChangeLog
+++ b/gdbserver/ChangeLog
@@ -1,3 +1,9 @@
+2020-07-17  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+       * tdesc.cc (allocate_target_description): Add header comment.
+       (target_desc_deleter::operator()): New function.
+       * tdesc.h (struct target_desc): Declare as final.
+
 2020-07-13  Simon Marchi  <simon.marchi@polymtl.ca>
 
        * server.cc (handle_query): Use std::vector of

diff --git a/gdbserver/tdesc.cc b/gdbserver/tdesc.cc
index d21688b932b875a4beda8ea0d6d6aca68f9af2cf..e639017cc31cac5ea2ba7ba76280fadb170455af 100644 (file)
--- a/gdbserver/tdesc.cc
+++ b/gdbserver/tdesc.cc
@@ -93,12 +93,22 @@ init_target_desc (struct target_desc *tdesc,
 #endif
 }
 
+/* See gdbsupport/tdesc.h.  */
+
 struct target_desc *
 allocate_target_description (void)
 {
   return new target_desc ();
 }
 
+/* See gdbsupport/tdesc.h.  */
+
+void
+target_desc_deleter::operator() (struct target_desc *target_desc) const
+{
+  delete target_desc;
+}
+
 #ifndef IN_PROCESS_AGENT
 
 static const struct target_desc default_description {};

diff --git a/gdbserver/tdesc.h b/gdbserver/tdesc.h
index f9ca478ff6db3813a80be5096ec2adb26640cec7..681de64d0aa313dc02515ca03c62e0d4105c631b 100644 (file)
--- a/gdbserver/tdesc.h
+++ b/gdbserver/tdesc.h
@@ -27,7 +27,7 @@
 /* A target description.  Inherit from tdesc_feature so that target_desc
    can be used as tdesc_feature.  */
 
-struct target_desc : tdesc_element
+struct target_desc final : tdesc_element
 {
   /* A vector of elements of register definitions that
      describe the inferior's register set.  */

diff --git a/gdbsupport/ChangeLog b/gdbsupport/ChangeLog
index e1b040d5827b79d8572cf93345c5633caf63442d..6b555cb97042f55e94b89d68db58f43c5675e328 100644 (file)
--- a/gdbsupport/ChangeLog
+++ b/gdbsupport/ChangeLog
@@ -1,3 +1,9 @@
+2020-07-17  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+       * tdesc.h (struct target_desc_deleter): Moved here
+       from gdb/target-descriptions.h, extend comment.
+       (target_desc_up): Likewise.
+
 2020-06-30  Tom Tromey  <tromey@adacore.com>
 
        PR build/26183:

diff --git a/gdbsupport/tdesc.h b/gdbsupport/tdesc.h
index 456e8e070b1230dc030cb90ed7cf68fcb5cba690..fdc2a6a370884b1c35bf7dfa10571329ff74d0ac 100644 (file)
--- a/gdbsupport/tdesc.h
+++ b/gdbsupport/tdesc.h
@@ -312,6 +312,20 @@ struct tdesc_feature : tdesc_element
 
 typedef std::unique_ptr<tdesc_feature> tdesc_feature_up;
 
+/* A deleter adapter for a target_desc.  There are different
+   implementations of this deleter class in gdb and gdbserver because even
+   though the target_desc name is shared between the two projects, the
+   actual implementations of target_desc are completely different.  */
+
+struct target_desc_deleter
+{
+  void operator() (struct target_desc *desc) const;
+};
+
+/* A unique pointer specialization that holds a target_desc.  */
+
+typedef std::unique_ptr<target_desc, target_desc_deleter> target_desc_up;
+
 /* Allocate a new target_desc.  */
 target_desc *allocate_target_description (void);