From fc958966e47f622d738088509bacd0573a9db2c7 Mon Sep 17 00:00:00 2001 From: Joel Brobecker Date: Tue, 14 Apr 2015 11:55:57 -0700 Subject: [PATCH] GDB crash trying to subscript array of variant record. Consider the following declarations: subtype Small_Type is Integer range 0 .. 10; type Record_Type (I : Small_Type := 0) is record S : String (1 .. I); end record; A2 : Array_Type := (1 => (I => 2, S => "AB"), 2 => (I => 1, S => "A"), 3 => (I => 0, S => <>)); Compiled with -fgnat-encodings=minimal, and trying to print one element of our array, valgrind reports an invalid memory access. On certain GNU/Linux boxes, malloc even reports it as well, and causes GDB to crash. (gdb) print a2(1) *** glibc detected *** /[...]/gdb: malloc(): memory corruption: 0x0a30ba48 *** [crash] The invalid memory access occurs because of a simple buffer overflow in ada_value_primitive_packed_val. When this function is called, it is given a bit_size of 128 (or 16 bytes), which corresponds to the stride of our array. But the actual size of each element depends on its value. In particular, A2(1) is a record whose size is only 6 bytes. What happens in our example is that we start building a new value (v) where the element is to be unpacked, with any of its dynamic properties getting resolved as well. We then unpack the data into this value's buffer: unpacked = (unsigned char *) value_contents (v); [...] nsrc = len; [...] while (nsrc > 0) { [...] unpacked[targ] = accum & ~(~0L << HOST_CHAR_BIT); [...] targ += delta; [...] nsrc -= 1; [...] } In the loop above, targ starts at zero (for LE architectures), and len is 16. With delta being +1, we end up iterating 16 times, writing 16 bytes into a 6-bytes buffer. This patch fixes the issue by adjusting BIT_SIZE and recomputing LEN after having resolved our type if the resolved type turns out to be smaller than bit_size. gdb/ChangeLog: * ada-lang.c (ada_value_primitive_packed_val): Recompute BIT_SIZE and LEN if the size of the resolved type is smaller than BIT_SIZE * HOST_CHAR_BIT. --- gdb/ChangeLog | 6 ++++++ gdb/ada-lang.c | 11 +++++++++++ 2 files changed, 17 insertions(+) diff --git a/gdb/ChangeLog b/gdb/ChangeLog index 7c61032914..990bad71c5 100644 --- a/gdb/ChangeLog +++ b/gdb/ChangeLog @@ -1,3 +1,9 @@ +2015-05-05 Joel Brobecker + + * ada-lang.c (ada_value_primitive_packed_val): Recompute + BIT_SIZE and LEN if the size of the resolved type is smaller + than BIT_SIZE * HOST_CHAR_BIT. + 2015-05-05 Joel Brobecker * ada-lang.c (ada_value_primitive_packed_val): Use a more diff --git a/gdb/ada-lang.c b/gdb/ada-lang.c index 42f84e47c7..d0aea26837 100644 --- a/gdb/ada-lang.c +++ b/gdb/ada-lang.c @@ -2419,6 +2419,17 @@ ada_value_primitive_packed_val (struct value *obj, const gdb_byte *valaddr, { v = value_at (type, value_address (obj) + offset); type = value_type (v); + if (TYPE_LENGTH (type) * HOST_CHAR_BIT < bit_size) + { + /* This can happen in the case of an array of dynamic objects, + where the size of each element changes from element to element. + In that case, we're initially given the array stride, but + after resolving the element type, we find that its size is + less than this stride. In that case, adjust bit_size to + match TYPE's length, and recompute LEN accordingly. */ + bit_size = TYPE_LENGTH (type) * HOST_CHAR_BIT; + len = TYPE_LENGTH (type) + (bit_offset + HOST_CHAR_BIT - 1) / 8; + } bytes = (unsigned char *) alloca (len); read_memory (value_address (v), bytes, len); } -- 2.34.1