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Re: [PATCH 2/2] Documentation and testcase
- From: Pedro Alves <palves at redhat dot com>
- To: Sergio Durigan Junior <sergiodj at redhat dot com>, GDB Patches <gdb-patches at sourceware dot org>
- Date: Fri, 20 Mar 2015 19:46:13 +0000
- Subject: Re: [PATCH 2/2] Documentation and testcase
- Authentication-results: sourceware.org; auth=none
- References: <1426807358-18295-1-git-send-email-sergiodj at redhat dot com> <1426807358-18295-3-git-send-email-sergiodj at redhat dot com>
On 03/19/2015 11:22 PM, Sergio Durigan Junior wrote:
> ---
> gdb/doc/gdb.texinfo | 33 ++++++++
> gdb/testsuite/gdb.base/coredump-filter.c | 61 ++++++++++++++
> gdb/testsuite/gdb.base/coredump-filter.exp | 128 +++++++++++++++++++++++++++++
> 3 files changed, 222 insertions(+)
> create mode 100644 gdb/testsuite/gdb.base/coredump-filter.c
> create mode 100644 gdb/testsuite/gdb.base/coredump-filter.exp
>
> diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
> index 552da31..5382e91 100644
> --- a/gdb/doc/gdb.texinfo
> +++ b/gdb/doc/gdb.texinfo
> @@ -10952,6 +10952,39 @@ specified, the file name defaults to @file{core.@var{pid}}, where
>
> Note that this command is implemented only for some systems (as of
> this writing, @sc{gnu}/Linux, FreeBSD, Solaris, and S390).
> +
> +On @sc{gnu}/Linux, this command can take into account the value of the
> +file @file{/proc/@var{pid}/coredump_filter} when generating the core
> +dump (@pxref{set use-coredump-filter}).
> +
> +@kindex set use-coredump-filter
> +@anchor{set use-coredump-filter}
> +@item set use-coredump-filter on
> +@itemx set use-coredump-filter off
> +Enable or disable the use of the file
> +@file{/proc/@var{pid}/coredump_filter} when generating core dump
> +files. This file is used by the Linux kernel to decide what types of
> +memory mappings will be dumped or ignored when generating a core dump
> +file. @var{pid} is the process ID of a currently running process.
> +
> +
> +To make use of this feature, you have to write in the
> +@file{/proc/@var{pid}/coredump_filter} file a value, in hexadecimal,
> +which is a bit mask representing the memory mapping types. If a bit
> +is set in the bit mask, then the memory mappings of the corresponding
> +types will be dumped; otherwise, they will be ignored. For more
> +information about the bits that can be set in the
> +@file{/proc/@var{pid}/coredump_filter} file, please refer to the
> +manpage of @code{core(5)}.
Might be good to mention that the settings are inherited by child
processes. Reading this, I thought "wow, do I really need to
set every time I'm debugging a new pid/process?"
> + # The variables are 'char', and using it here would be OK because
> + # GDB actually reads the contents of the memory (i.e., it
> + # dereferences the pointer). However, to make it clear that we
> + # are interested not in the pointer itself, but in the memory it
> + # points to, we are using '*(unsigned int *)'.
> + gdb_test "print *(unsigned int *) $addr($var)" "\(\\\$$decimal = <error: \)?Cannot access memory at address $hex\(>\)?" \
> + "printing $var when core is loaded (should not work)"
> + gdb_test "print/x *(unsigned int *) $addr($working_var)" " = $working_value.*" \
> + "print/x *$working_var ( = $working_value)"
This comment still gave me pause. The variables are
'char *' not 'char':
char *private_anon, *shared_anon;
char *dont_dump;
so I guess you're referring to the issue that plain "print" would
assume they are strings and thus deference the pointer, right?
I honestly think that all that just distracts from what
we're doing. Why not just:
# Access the memory the addresses point to.
gdb_test "print *(char *) $addr($var)" "\(\\\$$decimal = <error: \)?Cannot access memory at address $hex\(>\)?" \
I would never ever think to do:
gdb_test "print (char *) $addr($var)"
to test the contents of what addr points to. IOW, reading
# Access the memory the addresses point to.
gdb_test "print *(char *) $addr($var)" ...
I'd never really wonder why the leftmost '*' is in there. It's super
obvious.
Maybe even throw in an /x for super clarity:
gdb_test "print /x *(char *) $addr($var)" ...
> +set all_corefiles { { "non-Private-Anonymous" "0x7e" \
> + $non_private_anon_core \
> + "private_anon" \
> + "shared_anon" "0x22" }
> + { "non-Shared-Anonymous" "0x7d" \
> + $non_shared_anon_core "shared_anon" \
> + "private_anon" "0x11" }
> + { "DoNotDump" "0x33" \
> + $dont_dump_core "dont_dump" \
> + "shared_anon" "0x22" } }
Does this cover the case of making sure we don't dump file-based
regions? That's important.
If not (I assume not), we could test that by loading the core
into gdb, but _not_ the program, and then disassembling a function's
address. It should fail. Then load the program and disassemble
again. It should work now. Or something along those lines.
Thanks,
Pedro Alves