Re: [Crash-utility] [PATCH 1/2] Change functions in extensions/echo.c to be static

-----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(a)nec.com&gt; 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(a)redhat.com&gt; > --- > 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