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* [binutils-gdb] gdbserver: special case target_write_memory len==0
@ 2022-04-14 19:23 Pedro Alves
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From: Pedro Alves @ 2022-04-14 19:23 UTC (permalink / raw)
To: gdb-cvs
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=366e3746c572c2c78454761e62fa9181cba413ca
commit 366e3746c572c2c78454761e62fa9181cba413ca
Author: Pedro Alves <pedro@palves.net>
Date: Thu Mar 31 22:04:42 2022 +0100
gdbserver: special case target_write_memory len==0
The next patch in this series adds a common helper routine for both
memory reads and writes, like this:
static int
proc_xfer_memory (CORE_ADDR memaddr, unsigned char *readbuf,
const gdb_byte *writebuf, int len)
{
gdb_assert ((readbuf == nullptr) != (writebuf == nullptr));
...
}
int
linux_process_target::read_memory (CORE_ADDR memaddr,
unsigned char *myaddr, int len)
{
return proc_xfer_memory (memaddr, myaddr, nullptr, len);
}
linux_process_target::write_memory (CORE_ADDR memaddr,
const unsigned char *myaddr, int len)
{
return proc_xfer_memory (memaddr, nullptr, myaddr, len);
}
Surprisingly, the assertion fails. That happens because it can happen
that target_write_memory is called with LEN==0, due to this in
gdb/remote.c:
/* Determine whether the remote target supports binary downloading.
This is accomplished by sending a no-op memory write of zero length
to the target at the specified address. (...) */
void
remote_target::check_binary_download (CORE_ADDR addr)
{
...
p = rs->buf.data ();
*p++ = 'X';
p += hexnumstr (p, (ULONGEST) addr);
*p++ = ',';
p += hexnumstr (p, (ULONGEST) 0);
*p++ = ':';
*p = '\0';
In this scenario, in gdbserver's target_write_memory, the "myaddr"
argument of the_target->write_memory is passed the data() of a local
gdb::byte_vector (which is a specialized std::vector). It's valid for
std::vector::data() to return NULL when the vector is empty.
This commit adds an early return to target_write_memory to avoid
target backends having to care about this. For good measure, do the
same on the read side, in read_inferior_memory.
Change-Id: Iac8f04fcf99014c624ef4036bd318ca1771ad491
Diff:
---
gdbserver/target.cc | 17 +++++++++++++++--
1 file changed, 15 insertions(+), 2 deletions(-)
diff --git a/gdbserver/target.cc b/gdbserver/target.cc
index 5009146d663..e9d1e1aa38c 100644
--- a/gdbserver/target.cc
+++ b/gdbserver/target.cc
@@ -124,8 +124,14 @@ done_accessing_memory (void)
int
read_inferior_memory (CORE_ADDR memaddr, unsigned char *myaddr, int len)
{
- int res;
- res = the_target->read_memory (memaddr, myaddr, len);
+ /* At the time of writing, GDB only sends write packets with LEN==0,
+ not read packets (see comment in target_write_memory), but it
+ doesn't hurt to prevent problems if it ever does, or we're
+ connected to some client other than GDB that does. */
+ if (len == 0)
+ return 0;
+
+ int res = the_target->read_memory (memaddr, myaddr, len);
check_mem_read (memaddr, myaddr, len);
return res;
}
@@ -152,6 +158,13 @@ int
target_write_memory (CORE_ADDR memaddr, const unsigned char *myaddr,
ssize_t len)
{
+ /* GDB may send X packets with LEN==0, for probing packet support.
+ If we let such a request go through, then buffer.data() below may
+ return NULL, which may confuse target implementations. Handle it
+ here to avoid lower levels having to care about this case. */
+ if (len == 0)
+ return 0;
+
/* Make a copy of the data because check_mem_write may need to
update it. */
gdb::byte_vector buffer (myaddr, myaddr + len);
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