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

Fabian Kilger
2025-05-24T20:26:32Z

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Thanks for reviewing, I'll be including these in v2. I have questions
regarding some style recommendations, where I feel like I'd be breaking
the style of the remaining file.

On 5/23/25 20:14, Andrew Burgess wrote:
> Fabian Kilger <kilger@sec.in.tum.de> writes:
>
> Thanks for working on this. This mostly looks good, I have just a few
> style issues that I think should be fixed, but it's all pretty minor.
>
>
>> 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 ++++
>> 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
>
> Add an empty line after the comment please to match the other functions
> in this file.
>
>> +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);
>
> There's no need to wrap this argument list.
>

I've tried copying the style of the other functions in the file. All
fileio_X functions wrap the argument list after linux_nat_fileio_pid_of.
Specifically also fileio_unlink, which has even less arguments. As such,
I feel like not wrapping would break with the style of the other
functions. Should I still not wrap it?

>> +
>> + 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);
>
> Space needed after 'stat'.
>
>> +
>> + return mnsh_return_intstr(sock, stat_ok, &sb,
>> + stat_ok == -1 ? 0 : sizeof (sb),
>> + errno);
>
> Space needed after 'mnsh_return_intstr'.
>
>> +}
>> +
>> /* 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);
>
> Space needed after 'mnsh_handle_stat'
>
>> + 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
>
> Move the empty line from before the comment to after the comment please.
>
>> +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;
>
> The declarations should be moved inline below.

Here I've also copied the style of the remaining functions: All
functions in that file declare the variables at the beginning in the
exact same fashion. See linux_mntns_open_cloexec right below it, for
example.

Best,
Fabian

>
>> +
>> + if (access == MNSH_FS_ERROR)
>> + return -1;
>> +
>> + if (access == MNSH_FS_DIRECT)
>> + return stat(filename, sb);
>
> Space needed after 'stat'.
>
>> +
>> + 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. */
>
> This comment doesn't need to wrap.
>
> Thanks,
> Andrew
>
>
>
>> +
>> +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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