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

Fabian Kilger
2025-05-11T15:02:24Z

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
+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