gdb: check max-value-size when reading strings for printf

I noticed that the printf code for strings, printf_c_string and
printf_wide_c_string, don't take max-value-size into account, but do
load a complete string from the inferior into a GDB buffer.

As such it would be possible for an badly behaved inferior to cause
GDB to try and allocate an excessively large buffer, potentially
crashing GDB, or at least causing GDB to swap lots, which isn't
great.

We already have a setting to protect against this sort of thing, the
'max-value-size'.  So this commit updates the two function mentioned
above to check the max-value-size and give an error if the
max-value-size is exceeded.

If the max-value-size is exceeded, I chose to continue reading
inferior memory to figure out how long the string actually is, we just
don't store the results.  The benefit of this is that when we give the
user an error we can tell the user how big the string actually is,
which would allow them to correctly adjust max-value-size, if that's
what they choose to do.

The default for max-value-size is 64k so there should be no user
visible changes after this commit, unless the user was previously
printing very large strings.  If that is the case then the user will
now need to increase max-value-size.

1 files changed, 6 insertions, 0 deletions

diff --git a/gdb/NEWS b/gdb/NEWS
index fd42864..b924834 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -78,6 +78,12 @@
   functionality is also available for dprintf when dprintf-style is
   'gdb'.
 
+* When the printf command requires a string to be fetched from the
+  inferior, GDB now uses the existing 'max-value-size' setting to the
+  limit the memory allocated within GDB.  The default 'max-value-size'
+  is 64k.  To print longer strings you should increase
+  'max-value-size'.
+
 * New commands
 
 maintenance print record-instruction [ N ]