gdb: New set/show max-value-size command.

For languages with dynamic types, an incorrect program, or uninitialised
variables within a program, could result in an incorrect, overly large
type being associated with a value.  Currently, attempting to print such
a variable will result in gdb trying to allocate an overly large buffer.

If this large memory allocation fails then the result can be gdb either
terminating, or (due to memory contention) becoming unresponsive for the
user.

A new user visible variable in gdb helps guard against such problems,
two new commands are available:

   set max-value-size
   show max-value-size

The 'max-value-size' is the maximum size of memory in bytes that gdb
will allocate for the contents of a value.  Any attempt to allocate a
value with a size greater than this will result in an error.  The
initial default for this limit is set at 64k, this is based on a similar
limit that exists within the ada specific code.

It is possible for the user to set max-value-size to unlimited, in which
case the old behaviour is restored.

gdb/ChangeLog:

	* value.c (max_value_size): New variable.
	(MIN_VALUE_FOR_MAX_VALUE_SIZE): New define.
	(show_max_value_size): New function.
	(check_type_length_before_alloc): New function.
	(allocate_value_contents): Call check_type_length_before_alloc.
	(set_value_enclosing_type): Likewise.
	(_initialize_values): Add set/show handler for max-value-size.
	* NEWS: Mention new set/show command.

gdb/doc/ChangeLog:

	* gdb.texinfo (Value Sizes): New section.
	(Data): Add the 'Value Sizes' node to the menu.

gdb/testsuite/ChangeLog:

	* gdb.base/max-value-size.c: New file.
	* gdb.base/max-value-size.exp: New file.
	* gdb.base/huge.exp: Disable max-value-size for this test.

2 files changed, 47 insertions, 0 deletions

diff --git a/gdb/doc/ChangeLog b/gdb/doc/ChangeLog
index da49549..176fcb1 100644
--- a/gdb/doc/ChangeLog
+++ b/gdb/doc/ChangeLog
@@ -1,3 +1,8 @@
+2016-02-01  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* gdb.texinfo (Value Sizes): New section.
+	(Data): Add the 'Value Sizes' node to the menu.
+
 2016-01-26  John Baldwin  <jhb@FreeBSD.org>
 
 	* gdb.texinfo (Debugging Output): Fix subject verb disagreements.
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index de3fe57..c1529a5 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -8636,6 +8636,7 @@ being passed the type of @var{arg} as the argument.
                                 character set than GDB does
 * Caching Target Data::         Data caching for targets
 * Searching Memory::            Searching memory for a sequence of bytes
+* Value Sizes::                 Managing memory allocated for values
 @end menu
 
 @node Expressions
@@ -11711,6 +11712,47 @@ $1 = 1
 $2 = (void *) 0x8049560
 @end smallexample
 
+@node Value Sizes
+@section Value Sizes
+
+Whenever @value{GDBN} prints a value memory will be allocated within
+@value{GDBN} to hold the contents of the value.  It is possible in
+some languages with dynamic typing systems, that an invalid program
+may indicate a value that is incorrectly large, this in turn may cause
+@value{GDBN} to try and allocate an overly large ammount of memory.
+
+@table @code
+@kindex set max-value-size
+@itemx set max-value-size @var{bytes}
+@itemx set max-value-size unlimited
+Set the maximum size of memory that @value{GDBN} will allocate for the
+contents of a value to @var{bytes}, trying to display a value that
+requires more memory than that will result in an error.
+
+Setting this variable does not effect values that have already been
+allocated within @value{GDBN}, only future allocations.
+
+There's a minimum size that @code{max-value-size} can be set to in
+order that @value{GDBN} can still operate correctly, this minimum is
+currently 16 bytes.
+
+The limit applies to the results of some subexpressions as well as to
+complete expressions.  For example, an expression denoting a simple
+integer component, such as @code{x.y.z}, may fail if the size of
+@var{x.y} is dynamic and exceeds @var{bytes}.  On the other hand,
+@value{GDBN} is sometimes clever; the expression @code{A[i]}, where
+@var{A} is an array variable with non-constant size, will generally
+succeed regardless of the bounds on @var{A}, as long as the component
+size is less than @var{bytes}.
+
+The default value of @code{max-value-size} is currently 64k.
+
+@kindex show max-value-size
+@item show max-value-size
+Show the maximum size of memory, in bytes, that @value{GDBN} will
+allocate for the contents of a value.
+@end table
+
 @node Optimized Code
 @chapter Debugging Optimized Code
 @cindex optimized code, debugging