-----Original Message-----
> If a same extension(Eg: extensions/trace.so) with two different names are loaded by
> "extend" command twice, it sometimes segfaults crash.
>
> It's because crash uses RTLD_NOW|RTLD_GLOBAL flags of dlopen to load an extension.
> RTDL_GLOBAL will make symbols defined by this shared object available for
> symbol resolution of subsequently loaded shared objects. So symbols in subsequently
> loaded extensions are overwritten by the former loaded one with a same name.
> Not only can it cause segfaults, but some other unexpected behaviours.

The phenomenon of the first paragraph is an example and a rare situation if
not on purpose, please let me change the order and generalize a bit more here:
---
The crash utility uses RTLD_NOW|RTLD_GLOBAL flags ... with the same name.

This can cause unexpected behaviors when loading two extension modules that
have a symbol with the same name.  For example, we can reproduce a segmentation
violation by loading the current trace.so extension module with two different
names.
---

I'll edit when applying.  Otherwise, the patch looks good to me.
Please wait for another ack.

Acked-by: Kazuhito Hagio <k-hagio-ab@nec.com>

Thanks,
Kazu

>
> This patch changes functions within extensions/echo.c to be static, and documents
> the issue in code comments, for extensions developers who takes echo.c as reference.
>
> Signed-off-by: Tao Liu <ltao@redhat.com>
> ---
>  extensions/echo.c | 22 ++++++++++++++--------
>  1 file changed, 14 insertions(+), 8 deletions(-)
>
> diff --git a/extensions/echo.c b/extensions/echo.c
> index ff1e09a..f4bb88f 100644
> --- a/extensions/echo.c
> +++ b/extensions/echo.c
> @@ -17,19 +17,25 @@
>
>  #include "defs.h"      /* From the crash source top-level directory */
>
> -void echo_init(void);    /* constructor function */
> -void echo_fini(void);    /* destructor function (optional) */
> +static void echo_init(void);    /* constructor function */
> +static void echo_fini(void);    /* destructor function (optional) */
>
> -void cmd_echo(void);     /* Declare the commands and their help data. */
> -char *help_echo[];
> +static void cmd_echo(void);     /* Declare the commands and their help data. */
> +static char *help_echo[];
>
> +/*
> + * Please making the functions and global variables static within your
> + * extension if you don't want to make them visiable to subsequently
> + * loaded extensions. Otherwise, non-static symbols within 2 extensions and
> + * have a same name can cause confliction.
> + */
>  static struct command_table_entry command_table[] = {
>          { "echo", cmd_echo, help_echo, 0},          /* One or more commands, */
>          { NULL },                                     /* terminated by NULL, */
>  };
>
>
> -void __attribute__((constructor))
> +static void __attribute__((constructor))
>  echo_init(void) /* Register the command set. */
>  {
>          register_extension(command_table);
> @@ -39,7 +45,7 @@ echo_init(void) /* Register the command set. */
>   *  This function is called if the shared object is unloaded.
>   *  If desired, perform any cleanups here.
>   */
> -void __attribute__((destructor))
> +static void __attribute__((destructor))
>  echo_fini(void) { }
>
>
> @@ -49,7 +55,7 @@ echo_fini(void) { }
>   *  other crash commands for usage of the myriad of utility routines available
>   *  to accomplish what your task.
>   */
> -void
> +static void
>  cmd_echo(void)
>  {
>          int c;
> @@ -94,7 +100,7 @@ cmd_echo(void)
>   *
>   */
>
> -char *help_echo[] = {
> +static char *help_echo[] = {
>          "echo",                        /* command name */
>          "echoes back its arguments",   /* short description */
>          "arg ...",                     /* argument synopsis, or " " if none */
> --
> 2.29.2