gdbstub: Add support for info proc mappings

Currently the GDB's generate-core-file command doesn't work well with
qemu-user: the resulting dumps are huge [1] and at the same time
incomplete (argv and envp are missing). The reason is that GDB has no
access to proc mappings and therefore has to fall back to using
heuristics for discovering them. This is, in turn, because qemu-user
does not implement the Host I/O feature of the GDB Remote Serial
Protocol.

Implement vFile:{open,close,pread,readlink} and also
qXfer:exec-file:read+. With that, generate-core-file begins to work on
aarch64 and s390x.

[1] https://sourceware.org/pipermail/gdb-patches/2023-May/199432.html

Co-developed-by: Dominik 'Disconnect3d' Czarnota <dominik.b.czarnota@gmail.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20230621203627.1808446-7-iii@linux.ibm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230630180423.558337-37-alex.bennee@linaro.org>

gdbstub/gdbstub.c

@@ -1327,6 +1327,36 @@ static const GdbCmdParseEntry gdb_v_commands_table[] = {
         .cmd = "Kill;",
         .cmd_startswith = 1
     },
+#ifdef CONFIG_USER_ONLY
+    /*
+     * Host I/O Packets. See [1] for details.
+     * [1] https://sourceware.org/gdb/onlinedocs/gdb/Host-I_002fO-Packets.html
+     */
+    {
+        .handler = gdb_handle_v_file_open,
+        .cmd = "File:open:",
+        .cmd_startswith = 1,
+        .schema = "s,L,L0"
+    },
+    {
+        .handler = gdb_handle_v_file_close,
+        .cmd = "File:close:",
+        .cmd_startswith = 1,
+        .schema = "l0"
+    },
+    {
+        .handler = gdb_handle_v_file_pread,
+        .cmd = "File:pread:",
+        .cmd_startswith = 1,
+        .schema = "l,L,L0"
+    },
+    {
+        .handler = gdb_handle_v_file_readlink,
+        .cmd = "File:readlink:",
+        .cmd_startswith = 1,
+        .schema = "s0"
+    },
+#endif
 };
 
 static void handle_v_commands(GArray *params, void *user_ctx)
@@ -1472,11 +1502,14 @@ static void handle_query_supported(GArray *params, void *user_ctx)
             ";ReverseStep+;ReverseContinue+");
     }
 
-#if defined(CONFIG_USER_ONLY) && defined(CONFIG_LINUX)
+#if defined(CONFIG_USER_ONLY)
+#if defined(CONFIG_LINUX)
     if (gdbserver_state.c_cpu->opaque) {
         g_string_append(gdbserver_state.str_buf, ";qXfer:auxv:read+");
     }
 #endif
+    g_string_append(gdbserver_state.str_buf, ";qXfer:exec-file:read+");
+#endif
 
     if (params->len &&
         strstr(get_param(params, 0)->data, "multiprocess+")) {
@@ -1615,13 +1648,21 @@ static const GdbCmdParseEntry gdb_gen_query_table[] = {
         .cmd_startswith = 1,
         .schema = "s:l,l0"
     },
-#if defined(CONFIG_USER_ONLY) && defined(CONFIG_LINUX)
+#if defined(CONFIG_USER_ONLY)
+#if defined(CONFIG_LINUX)
     {
         .handler = gdb_handle_query_xfer_auxv,
         .cmd = "Xfer:auxv:read::",
         .cmd_startswith = 1,
         .schema = "l,l0"
     },
+#endif
+    {
+        .handler = gdb_handle_query_xfer_exec_file,
+        .cmd = "Xfer:exec-file:read:",
+        .cmd_startswith = 1,
+        .schema = "l:l,l0"
+    },
 #endif
     {
         .handler = gdb_handle_query_attached,

gdbstub/internals.h

@@ -189,6 +189,11 @@ typedef union GdbCmdVariant {
 void gdb_handle_query_rcmd(GArray *params, void *user_ctx); /* softmmu */
 void gdb_handle_query_offsets(GArray *params, void *user_ctx); /* user */
 void gdb_handle_query_xfer_auxv(GArray *params, void *user_ctx); /*user */
+void gdb_handle_v_file_open(GArray *params, void *user_ctx); /* user */
+void gdb_handle_v_file_close(GArray *params, void *user_ctx); /* user */
+void gdb_handle_v_file_pread(GArray *params, void *user_ctx); /* user */
+void gdb_handle_v_file_readlink(GArray *params, void *user_ctx); /* user */
+void gdb_handle_query_xfer_exec_file(GArray *params, void *user_ctx); /* user */
 
 void gdb_handle_query_attached(GArray *params, void *user_ctx); /* both */
 

gdbstub/user-target.c

@@ -11,6 +11,10 @@
 #include "exec/gdbstub.h"
 #include "qemu.h"
 #include "internals.h"
+#ifdef CONFIG_LINUX
+#include "linux-user/loader.h"
+#include "linux-user/qemu.h"
+#endif
 
 /*
  * Map target signal numbers to GDB protocol signal numbers and vice
@@ -281,3 +285,136 @@ void gdb_handle_query_xfer_auxv(GArray *params, void *user_ctx)
                       gdbserver_state.str_buf->len, true);
 }
 #endif
+
+static const char *get_filename_param(GArray *params, int i)
+{
+    const char *hex_filename = get_param(params, i)->data;
+    gdb_hextomem(gdbserver_state.mem_buf, hex_filename,
+                 strlen(hex_filename) / 2);
+    g_byte_array_append(gdbserver_state.mem_buf, (const guint8 *)"", 1);
+    return (const char *)gdbserver_state.mem_buf->data;
+}
+
+static void hostio_reply_with_data(const void *buf, size_t n)
+{
+    g_string_printf(gdbserver_state.str_buf, "F%zx;", n);
+    gdb_memtox(gdbserver_state.str_buf, buf, n);
+    gdb_put_packet_binary(gdbserver_state.str_buf->str,
+                          gdbserver_state.str_buf->len, true);
+}
+
+void gdb_handle_v_file_open(GArray *params, void *user_ctx)
+{
+    const char *filename = get_filename_param(params, 0);
+    uint64_t flags = get_param(params, 1)->val_ull;
+    uint64_t mode = get_param(params, 2)->val_ull;
+
+#ifdef CONFIG_LINUX
+    int fd = do_guest_openat(gdbserver_state.g_cpu->env_ptr, 0, filename,
+                             flags, mode, false);
+#else
+    int fd = open(filename, flags, mode);
+#endif
+    if (fd < 0) {
+        g_string_printf(gdbserver_state.str_buf, "F-1,%d", errno);
+    } else {
+        g_string_printf(gdbserver_state.str_buf, "F%d", fd);
+    }
+    gdb_put_strbuf();
+}
+
+void gdb_handle_v_file_close(GArray *params, void *user_ctx)
+{
+    int fd = get_param(params, 0)->val_ul;
+
+    if (close(fd) == -1) {
+        g_string_printf(gdbserver_state.str_buf, "F-1,%d", errno);
+        gdb_put_strbuf();
+        return;
+    }
+
+    gdb_put_packet("F00");
+}
+
+void gdb_handle_v_file_pread(GArray *params, void *user_ctx)
+{
+    int fd = get_param(params, 0)->val_ul;
+    size_t count = get_param(params, 1)->val_ull;
+    off_t offset = get_param(params, 2)->val_ull;
+
+    size_t bufsiz = MIN(count, BUFSIZ);
+    g_autofree char *buf = g_try_malloc(bufsiz);
+    if (buf == NULL) {
+        gdb_put_packet("E12");
+        return;
+    }
+
+    ssize_t n = pread(fd, buf, bufsiz, offset);
+    if (n < 0) {
+        g_string_printf(gdbserver_state.str_buf, "F-1,%d", errno);
+        gdb_put_strbuf();
+        return;
+    }
+    hostio_reply_with_data(buf, n);
+}
+
+void gdb_handle_v_file_readlink(GArray *params, void *user_ctx)
+{
+    const char *filename = get_filename_param(params, 0);
+
+    g_autofree char *buf = g_try_malloc(BUFSIZ);
+    if (buf == NULL) {
+        gdb_put_packet("E12");
+        return;
+    }
+
+#ifdef CONFIG_LINUX
+    ssize_t n = do_guest_readlink(filename, buf, BUFSIZ);
+#else
+    ssize_t n = readlink(filename, buf, BUFSIZ);
+#endif
+    if (n < 0) {
+        g_string_printf(gdbserver_state.str_buf, "F-1,%d", errno);
+        gdb_put_strbuf();
+        return;
+    }
+    hostio_reply_with_data(buf, n);
+}
+
+void gdb_handle_query_xfer_exec_file(GArray *params, void *user_ctx)
+{
+    uint32_t pid = get_param(params, 0)->val_ul;
+    uint32_t offset = get_param(params, 1)->val_ul;
+    uint32_t length = get_param(params, 2)->val_ul;
+
+    GDBProcess *process = gdb_get_process(pid);
+    if (!process) {
+        gdb_put_packet("E00");
+        return;
+    }
+
+    CPUState *cpu = gdb_get_first_cpu_in_process(process);
+    if (!cpu) {
+        gdb_put_packet("E00");
+        return;
+    }
+
+    TaskState *ts = cpu->opaque;
+    if (!ts || !ts->bprm || !ts->bprm->filename) {
+        gdb_put_packet("E00");
+        return;
+    }
+
+    size_t total_length = strlen(ts->bprm->filename);
+    if (offset > total_length) {
+        gdb_put_packet("E00");
+        return;
+    }
+    if (offset + length > total_length) {
+        length = total_length - offset;
+    }
+
+    g_string_printf(gdbserver_state.str_buf, "l%.*s", length,
+                    ts->bprm->filename + offset);
+    gdb_put_strbuf();
+}