From: Pedro Alves <pedro@palves.net>
To: Keith Seitz <keiths@redhat.com>, gdb-patches@sourceware.org
Subject: Re: [PATCH] linux_nat_target::xfer_partial: Fallback to ptrace
Date: Fri, 20 May 2022 19:51:06 +0100 [thread overview]
Message-ID: <23154482-133e-8bfe-6d14-17f7e79b716b@palves.net> (raw)
In-Reply-To: <20220512181557.2093666-1-keiths@redhat.com>
On 2022-05-12 19:15, Keith Seitz via Gdb-patches wrote:
> Commit 05c06f318fd9a112529dfc313e6512b399a645e4 enabled GDB
> to access memory while threads are running. It did this by accessing
> /proc/PID/task/LWP/mem.
>
> Unfortunatley, this interface is not implemented for writing in older kernels
Unfortunatley -> Unfortunately
> (such as RHEL6). This means that GDB is unable to insert breakpoints on
> these hosts:
>
> $ ./gdb -q gdb -ex start
> Reading symbols from gdb...
> Temporary breakpoint 1 at 0x40fdd5: file ../../src/gdb/gdb.c, line 28.
> Starting program: /home/rhel6/fsf/linux/gdb/gdb
> Warning:
> Cannot insert breakpoint 1.
> Cannot access memory at address 0x40fdd5
>
> (gdb)
>
Oh man. I thought such kernels were already older than the oldest version
we support, but looks like not. :-/ I don't suppose you could instead
convince the kernel team to backport the patches that made /proc/pid/mem
writable (https://lore.kernel.org/lkml/20110314151320.GG21770@outflux.net/T/).. :-P
Both gdb and gdbserver are now relying on this to access memory of running threads.
This never worked for gdb, but it did for gdbserver, by stopping all threads temporarily.
I would really-really-really prefer not to add that code back for ancient
kernels...
> --- a/gdb/linux-nat.c
> +++ b/gdb/linux-nat.c
> @@ -3706,8 +3706,12 @@ linux_nat_target::xfer_partial (enum target_object object,
> if (addr_bit < (sizeof (ULONGEST) * HOST_CHAR_BIT))
> offset &= ((ULONGEST) 1 << addr_bit) - 1;
>
> - return linux_proc_xfer_memory_partial (readbuf, writebuf,
> - offset, len, xfered_len);
> + enum target_xfer_status xfer
> + = linux_proc_xfer_memory_partial (readbuf, writebuf,
> + offset, len, xfered_len);
> + if (xfer != TARGET_XFER_EOF)
> + return xfer;
> + /* Fallthrough to ptrace. */
Seems fine, but I'd like a comment here giving a hint that we'll be able to
remove this once we stop supporting such old kernels. Something like:
/* Fallthrough to ptrace. /proc/pid/mem wasn't writable before Linux 2.6.39. */
I got that number by finding commit 198214a7ee50, and looking at git tag --contains 198214a7ee50.
AFAICT, RHEL 6 is on 2.6.32.
next prev parent reply other threads:[~2022-05-20 18:51 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-12 18:15 Keith Seitz
2022-05-20 18:51 ` Pedro Alves [this message]
2022-05-24 18:56 ` Keith Seitz
2022-05-25 13:41 ` Pedro Alves
2022-06-03 15:18 Keith Seitz
2022-07-21 15:03 ` Keith Seitz
2022-07-21 20:07 ` Pedro Alves
2022-07-26 17:24 ` Keith Seitz
2022-07-26 19:16 ` Pedro Alves
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