Re: [PATCH 1/2][PR GDB/32956] gdb: implement linux namespace support for fileio_stat

Andrew Burgess
2025-06-11T09:51:37Z

Fabian Kilger <kilger@sec.in.tum.de> writes:

> While looking at it, I've noticed all implementations of stat functions
> actually use lstat and not stat. This maybe should be modified in the
> namespace case as well and I'll be changing this for v2. However, I
> could not directly find a rationale behind why *_stat functions call
> lstat instead. Possibly, it might make sense renaming the
> target_fileio_stat to target_fileio_lstat as well, though this would be
> an independent change.

Without digging into the history I say for sure why it's lstat over
stat.

I agree with you that renaming the API might make some sense, though I
suspect we're really a little stuck, the remote protocol packet is
called 'stat', which is unfortunate, we cannot really change that.

I do agree that within the namespace code we should probably use lstat
in order to be consistent.

Thanks,
Andrew

>
> Best,
> Fabian
>
> On 5/24/25 21:43, Fabian Kilger wrote:
>> Hi Andrew,
>>
>> you're right, I'm directly using GDB and wasn't aware of that gdbserver
>> module. I'll be adding it to the patch for v2.
>>
>> Best,
>> Fabian
>>
>> On 5/24/25 12:46, Andrew Burgess wrote:
>>> Fabian Kilger <kilger@sec.in.tum.de> writes:
>>>
>>>> The new algorithm to look for a build-id-based debug file
>>>> (introduced by commit 22836ca88591ac7efacf06d5b6db191763fd8aba)
>>>> makes use of fileio_stat. As fileio_stat was not supported by
>>>> linux-namespace.c, all stat calls would be performed on the host
>>>> and not inside the namespace
>>>>
>>>> ---
>>>> gdb/linux-nat.c | 14 ++++++++
>>>> gdb/linux-nat.h | 3 ++
>>>> gdb/nat/linux-namespaces.c | 71 ++++++++++++++++++++++++++++++++++++++
>>>> gdb/nat/linux-namespaces.h | 6 ++++
>>>
>>> Thinking about this some more, I realised that this is missing the
>>> gdbserver related changes.
>>>
>>> If you search in the gdbserver/ directory for how multifs_readlink is
>>> used then this will give a pretty good idea for how a new multifs_stat
>>> should be added.
>>>
>>> For manual testing things will be pretty similar to testing GDB. On the
>>> same host as your container, but outside of the container, start
>>> gdbserver:
>>>
>>> $ gdbserver --multi --once :54321
>>>
>>> Then on the same host, start GDB, and within GDB:
>>>
>>> (gdb) target extended-remote :54321
>>> (gdb) attach PID
>>>
>>> I'm assuming that currently you are just attaching directly from GDB to
>>> a process within the container?
>>>
>>> Thanks,
>>> Andrew
>>>
>>>> 4 files changed, 94 insertions(+)
>>>>
>>>> diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
>>>> index 3f252370c7b..478a7977c4d 100644
>>>> --- a/gdb/linux-nat.c
>>>> +++ b/gdb/linux-nat.c
>>>> @@ -4585,6 +4585,20 @@ linux_nat_target::fileio_open (struct inferior *inf, const char *filename,
>>>> return fd;
>>>> }
>>>>
>>>> +/* Implementation of to_fileio_stat. */
>>>> +int
>>>> +linux_nat_target::fileio_stat (struct inferior *inf, const char *filename,
>>>> + struct stat *sb, fileio_error *target_errno)
>>>> +{
>>>> + int r = linux_mntns_stat (linux_nat_fileio_pid_of (inf),
>>>> + filename, sb);
>>>> +
>>>> + if (r == -1)
>>>> + *target_errno = host_to_fileio_error (errno);
>>>> +
>>>> + return r;
>>>> +}
>>>> +
>>>> /* Implementation of to_fileio_readlink. */
>>>>
>>>> std::optional<std::string>
>>>> diff --git a/gdb/linux-nat.h b/gdb/linux-nat.h
>>>> index b630b858e34..42d1ec142b3 100644
>>>> --- a/gdb/linux-nat.h
>>>> +++ b/gdb/linux-nat.h
>>>> @@ -108,6 +108,9 @@ class linux_nat_target : public inf_ptrace_target
>>>> const char *filename,
>>>> fileio_error *target_errno) override;
>>>>
>>>> + int fileio_stat (struct inferior *inf, const char *filename,
>>>> + struct stat *sb, fileio_error *target_errno) override;
>>>> +
>>>> int fileio_unlink (struct inferior *inf,
>>>> const char *filename,
>>>> fileio_error *target_errno) override;
>>>> diff --git a/gdb/nat/linux-namespaces.c b/gdb/nat/linux-namespaces.c
>>>> index 19a05eec905..aa74e9df950 100644
>>>> --- a/gdb/nat/linux-namespaces.c
>>>> +++ b/gdb/nat/linux-namespaces.c
>>>> @@ -233,6 +233,12 @@ enum mnsh_msg_type
>>>> MNSH_RET_INT. */
>>>> MNSH_REQ_SETNS,
>>>>
>>>> + /* A request that the helper call stat. The single
>>>> + argument (the filename) should be passed in BUF, and
>>>> + should include a terminating NUL character. The helper
>>>> + should respond with a MNSH_RET_INTSTR. */
>>>> + MNSH_REQ_STAT,
>>>> +
>>>> /* A request that the helper call open. Arguments should
>>>> be passed in BUF, INT1 and INT2. The filename (in BUF)
>>>> should include a terminating NUL character. The helper
>>>> @@ -283,6 +289,10 @@ mnsh_debug_print_message (enum mnsh_msg_type type,
>>>> debug_printf ("ERROR");
>>>> break;
>>>>
>>>> + case MNSH_REQ_STAT:
>>>> + debug_printf ("STAT");
>>>> + break;
>>>> +
>>>> case MNSH_REQ_SETNS:
>>>> debug_printf ("SETNS");
>>>> break;
>>>> @@ -514,6 +524,20 @@ mnsh_handle_setns (int sock, int fd, int nstype)
>>>> return mnsh_return_int (sock, result, errno);
>>>> }
>>>>
>>>> +
>>>> +/* Handle a MNSH_REQ_STAT message. Must be async-signal-safe. */
>>>> +
>>>> +static ssize_t
>>>> +mnsh_handle_stat(int sock, const char *filename)
>>>> +{
>>>> + struct stat sb;
>>>> + int stat_ok = stat(filename, &sb);
>>>> +
>>>> + return mnsh_return_intstr(sock, stat_ok, &sb,
>>>> + stat_ok == -1 ? 0 : sizeof (sb),
>>>> + errno);
>>>> +}
>>>> +
>>>> /* Handle a MNSH_REQ_OPEN message. Must be async-signal-safe. */
>>>>
>>>> static ssize_t
>>>> @@ -574,6 +598,11 @@ mnsh_main (int sock)
>>>> response = mnsh_handle_setns (sock, fd, int1);
>>>> break;
>>>>
>>>> + case MNSH_REQ_STAT:
>>>> + if (size > 0 && buf[size - 1] == '\0')
>>>> + response = mnsh_handle_stat(sock, buf);
>>>> + break;
>>>> +
>>>> case MNSH_REQ_OPEN:
>>>> if (size > 0 && buf[size - 1] == '\0')
>>>> response = mnsh_handle_open (sock, buf, int1, int2);
>>>> @@ -765,6 +794,10 @@ mnsh_maybe_mourn_peer (void)
>>>> mnsh_send_message (helper->sock, MNSH_REQ_OPEN, -1, flags, mode, \
>>>> filename, strlen (filename) + 1)
>>>>
>>>> +#define mnsh_send_stat(helper, filename) \
>>>> + mnsh_send_message (helper->sock, MNSH_REQ_STAT, -1, 0, 0, \
>>>> + filename, strlen (filename) + 1)
>>>> +
>>>> #define mnsh_send_unlink(helper, filename) \
>>>> mnsh_send_message (helper->sock, MNSH_REQ_UNLINK, -1, 0, 0, \
>>>> filename, strlen (filename) + 1)
>>>> @@ -945,6 +978,44 @@ linux_mntns_access_fs (pid_t pid)
>>>> return MNSH_FS_HELPER;
>>>> }
>>>>
>>>> +
>>>> +/* See nat/linux-namespaces.h. */
>>>> +int
>>>> +linux_mntns_stat (pid_t pid, const char *filename,
>>>> + struct stat *sb)
>>>> +{
>>>> + enum mnsh_fs_code access = linux_mntns_access_fs (pid);
>>>> + struct linux_mnsh *helper;
>>>> + int stat_ok, error;
>>>> + ssize_t size;
>>>> +
>>>> + if (access == MNSH_FS_ERROR)
>>>> + return -1;
>>>> +
>>>> + if (access == MNSH_FS_DIRECT)
>>>> + return stat(filename, sb);
>>>> +
>>>> + gdb_assert (access == MNSH_FS_HELPER);
>>>> +
>>>> + helper = linux_mntns_get_helper ();
>>>> +
>>>> + size = mnsh_send_stat (helper, filename);
>>>> + if (size < 0)
>>>> + return -1;
>>>> +
>>>> + size = mnsh_recv_intstr (helper, &stat_ok, &error, sb, sizeof (*sb));
>>>> +
>>>> + if (size < 0)
>>>> + {
>>>> + stat_ok = -1;
>>>> + errno = error;
>>>> + }
>>>> + else
>>>> + gdb_assert (stat_ok == -1 || size == sizeof (*sb));
>>>> +
>>>> + return stat_ok;
>>>> +}
>>>> +
>>>> /* See nat/linux-namespaces.h. */
>>>>
>>>> int
>>>> diff --git a/gdb/nat/linux-namespaces.h b/gdb/nat/linux-namespaces.h
>>>> index 4327292950b..825cb27eb2d 100644
>>>> --- a/gdb/nat/linux-namespaces.h
>>>> +++ b/gdb/nat/linux-namespaces.h
>>>> @@ -58,6 +58,12 @@ enum linux_ns_type
>>>>
>>>> extern int linux_ns_same (pid_t pid, enum linux_ns_type type);
>>>>
>>>> +/* Like stat(2), but in the mount namespace of process
>>>> + PID. */
>>>> +
>>>> +extern int linux_mntns_stat (pid_t pid, const char *filename,
>>>> + struct stat *sb);
>>>> +
>>>> /* Like gdb_open_cloexec, but in the mount namespace of process
>>>> PID. */
>>>>
>>>> --
>>>> 2.49.0
>>>
>>
>
> --
> Fabian Kilger, M.Sc.
> Wissenschaftlicher Mitarbeiter
>
> Technische Universität München
> TUM School of Computation, Information and Technology
> Chair of IT Security
>
> Boltzmannstraße 3
> 85748 Garching (bei München)
>
> Tel. +49 (0)89 289-18587
> Fax +49 (0)89 289-18579
>
> kilger@sec.in.tum.de
> www.sec.in.tum.de
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