Re: [PATCH 3/3] linux-nat: Exploit /proc//mem for writing

Pedro Alves
2017-03-13T20:05:10Z

On 03/06/2017 04:00 PM, Andreas Arnez wrote:
> So far linux_proc_xfer_partial refused to handle write requests. This is
> still based on the assumption that the Linux kernel does not support
> writes to /proc/<pid>/mem. That used to be true, but has changed with
> Linux 2.6.39 released in May 2011.

Hey, I had not noticed that. Awesome.

(There's also process_vm_readv / process_vm_writev.)

> This patch lifts this restriction and now exploits /proc/<pid>/mem for
> writing to inferior memory as well, if possible.
>
> gdb/ChangeLog:
>
> * linux-nat.c (linux_proc_xfer_partial): Handle write operations
> as well.
> ---
> gdb/linux-nat.c | 32 ++++++++++++++++----------------
> 1 file changed, 16 insertions(+), 16 deletions(-)
>
> diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
> index c58ed83..73ef2d4 100644
> --- a/gdb/linux-nat.c
> +++ b/gdb/linux-nat.c
> @@ -3978,10 +3978,9 @@ linux_child_pid_to_exec_file (struct target_ops *self, int pid)
> return linux_proc_pid_to_exec_file (pid);
> }
>
> -/* Implement the to_xfer_partial interface for memory reads using the /proc
> - filesystem. Because we can use a single read() call for /proc, this
> - can be much more efficient than banging away at PTRACE_PEEKTEXT,
> - but it doesn't support writes. */
> +/* Implement the to_xfer_partial target method using /proc/<pid>/mem.
> + Because we can use a single read/write call, this can be much more
> + efficient than banging away at PTRACE_PEEKTEXT. */
>
> static enum target_xfer_status
> linux_proc_xfer_partial (struct target_ops *ops, enum target_object object,
> @@ -3993,7 +3992,7 @@ linux_proc_xfer_partial (struct target_ops *ops, enum target_object object,
> int fd;
> char filename[64];
>
> - if (object != TARGET_OBJECT_MEMORY || !readbuf)
> + if (object != TARGET_OBJECT_MEMORY)
> return TARGET_XFER_EOF;
>
> /* Don't bother for one word. */
> @@ -4004,26 +4003,27 @@ linux_proc_xfer_partial (struct target_ops *ops, enum target_object object,
> thread. That requires some juggling, but is even faster. */
> xsnprintf (filename, sizeof filename, "/proc/%d/mem",
> ptid_get_pid (inferior_ptid));
> - fd = gdb_open_cloexec (filename, O_RDONLY | O_LARGEFILE, 0);
> + fd = gdb_open_cloexec (filename, ((readbuf ? O_RDONLY : O_WRONLY)
> + | O_LARGEFILE), 0);
> if (fd == -1)
> return TARGET_XFER_EOF;
>
> - /* If pread64 is available, use it. It's faster if the kernel
> - supports it (only one syscall), and it's 64-bit safe even on
> - 32-bit platforms (for instance, SPARC debugging a SPARC64
> - application). */
> + /* Use pread64/pwrite64 if available, since they save a syscall and can
> + handle 64-bit offsets even on 32-bit platforms (for instance, SPARC
> + debugging a SPARC64 application). */
> #ifdef HAVE_PREAD64
> - if (pread64 (fd, readbuf, len, offset) != len)
> + ret = (readbuf ? pread64 (fd, readbuf, len, offset)
> + : pwrite64 (fd, writebuf, len, offset));
> #else
> - if (lseek (fd, offset, SEEK_SET) == -1 || read (fd, readbuf, len) != len)
> + ret = lseek (fd, offset, SEEK_SET);
> + if (ret != -1)
> + ret = (readbuf ? read (fd, readbuf, len)
> + : write (fd, writebuf, len));
> #endif
> - ret = 0;
> - else
> - ret = len;
>
> close (fd);
>
> - if (ret == 0)
> + if (ret == -1 || ret == 0)
> return TARGET_XFER_EOF;

Are we sure we can't see partial reads/writes here?
I.e., seems like we lose the "read/pread64 (fd, readbuf, len) != len"
checks?

Thanks,
Pedro Alves