[PATCH 06/24] gdb: fix bugs in {get,put}_frame_register_bytes

Simon Marchi
2023-11-08T05:21:59Z

From: Simon Marchi <simon.marchi@efficios.com>

I found this only by inspection: the myaddr pointer in
{get,put}_frame_register_bytes is reset to `buffer.data ()` at each
iteration. This means that we will always use the bytes at the
beginning of `buffer` to read or write to the registers, instead of
progressing in `buffer`.

Fix this by re-writing the functions to chip away the beginning of the
buffer array_view as we progress in reading or writing the data.

These bugs was introduced almost 3 years ago [1], and yet nobody
complained. I'm wondering which architecture relies on that register
"overflow" behavior (reading or writing multiple consecutive registers
with one {get,put}_frame_register_bytes calls), and in which situation.
I find these functions a bit dangerous, if a caller mis-calculates
things, it could end up silently reading or writing to the next
register, even if it's not the intent.

If I could change it, I would prefer to have functions specifically made
for that ({get,put}_frame_register_bytes_consecutive or something like
that) and make {get,put}_frame_register_bytes only able to write within
a single register (which I presume represents most of the use cases of
the current {get,put}_frame_register_bytes). If a caller mis-calculates
things and an overflow occurs while calling
{get,put}_frame_register_bytes, it would hit an assert. The problem is
knowing which callers rely on the overflow behavior and which don't.

[1] https://gitlab.com/gnutools/binutils-gdb/-/commit/bdec2917b1e94c7198ba39919f45060067952f43

Change-Id: I43bd4a9f7fa8419d388a2b20bdc57d652688ddf8
---
gdb/frame.c | 63 +++++++++++++++++++----------------------------------
1 file changed, 23 insertions(+), 40 deletions(-)

diff --git a/gdb/frame.c b/gdb/frame.c
index afadb8ac4e73..b3d99163b4dc 100644
--- a/gdb/frame.c
+++ b/gdb/frame.c
@@ -1482,9 +1482,6 @@ get_frame_register_bytes (frame_info_ptr frame, int regnum,
int *optimizedp, int *unavailablep)
{
struct gdbarch *gdbarch = get_frame_arch (frame);
- int i;
- int maxsize;
- int numregs;

/* Skip registers wholly inside of OFFSET. */
while (offset >= register_size (gdbarch, regnum))
@@ -1495,9 +1492,9 @@ get_frame_register_bytes (frame_info_ptr frame, int regnum,

/* Ensure that we will not read beyond the end of the register file.
This can only ever happen if the debug information is bad. */
- maxsize = -offset;
- numregs = gdbarch_num_cooked_regs (gdbarch);
- for (i = regnum; i < numregs; i++)
+ int maxsize = -offset;
+ int numregs = gdbarch_num_cooked_regs (gdbarch);
+ for (int i = regnum; i < numregs; i++)
{
int thissize = register_size (gdbarch, i);

@@ -1506,20 +1503,15 @@ get_frame_register_bytes (frame_info_ptr frame, int regnum,
maxsize += thissize;
}

- int len = buffer.size ();
- if (len > maxsize)
+ if (buffer.size () > maxsize)
error (_("Bad debug information detected: "
- "Attempt to read %d bytes from registers."), len);
+ "Attempt to read %zu bytes from registers."), buffer.size ());

/* Copy the data. */
- while (len > 0)
+ while (!buffer.empty ())
{
- int curr_len = register_size (gdbarch, regnum) - offset;
-
- if (curr_len > len)
- curr_len = len;
-
- gdb_byte *myaddr = buffer.data ();
+ int curr_len = std::min<int> (register_size (gdbarch, regnum) - offset,
+ buffer.size ());

if (curr_len == register_size (gdbarch, regnum))
{
@@ -1527,8 +1519,8 @@ get_frame_register_bytes (frame_info_ptr frame, int regnum,
CORE_ADDR addr;
int realnum;

- frame_register (frame, regnum, optimizedp, unavailablep,
- &lval, &addr, &realnum, myaddr);
+ frame_register (frame, regnum, optimizedp, unavailablep, &lval,
+ &addr, &realnum, buffer.data ());
if (*optimizedp || *unavailablep)
return false;
}
@@ -1547,13 +1539,12 @@ get_frame_register_bytes (frame_info_ptr frame, int regnum,
return false;
}

- memcpy (myaddr, value->contents_all ().data () + offset,
- curr_len);
+ copy (value->contents_all ().slice (offset, curr_len),
+ buffer.slice (0, curr_len));
release_value (value);
}

- myaddr += curr_len;
- len -= curr_len;
+ buffer = buffer.slice (curr_len);
offset = 0;
regnum++;
}
@@ -1578,36 +1569,28 @@ put_frame_register_bytes (frame_info_ptr frame, int regnum,
regnum++;
}

- int len = buffer.size ();
/* Copy the data. */
- while (len > 0)
+ while (!buffer.empty ())
{
- int curr_len = register_size (gdbarch, regnum) - offset;
+ int curr_len = std::min<int> (register_size (gdbarch, regnum) - offset,
+ buffer.size ());

- if (curr_len > len)
- curr_len = len;
-
- const gdb_byte *myaddr = buffer.data ();
if (curr_len == register_size (gdbarch, regnum))
- {
- put_frame_register (frame, regnum, myaddr);
- }
+ put_frame_register (frame, regnum, buffer.data ());
else
{
- struct value *value
+ value *value
= frame_unwind_register_value (frame_info_ptr (frame->next),
regnum);
- gdb_assert (value != NULL);
+ gdb_assert (value != nullptr);

- memcpy ((char *) value->contents_writeable ().data () + offset,
- myaddr, curr_len);
- put_frame_register (frame, regnum,
- value->contents_raw ().data ());
+ copy (buffer.slice (0, curr_len),
+ value->contents_writeable ().slice (offset, curr_len));
+ put_frame_register (frame, regnum, value->contents_raw ().data ());
release_value (value);
}

- myaddr += curr_len;
- len -= curr_len;
+ buffer = buffer.slice (curr_len);
offset = 0;
regnum++;
}
--
2.42.1