[Mesa-stable] [Mesa-dev] [PATCH v2] dri megadriver_stub: provide compatibility with older DRI loader

[Matt Turner]

On Fri, Dec 6, 2013 at 7:11 PM, Jordan Justen <jordan.l.justen at intel.com> wrote:
> To help the transition period when DRI loaders are being updated
> to support the newer __driDriverExtensions_foo mechanism,
> we populate __DRIextension with the extensions returned
> by __driDriverExtensions_foo during a library contructor
> function.
>
> We find the driver foo's name by using the dladdr function
> which gives the path of the dynamic library's name that
> was being loaded.
>
> v2:
>  * dladdr on public symbol __driDriverExtensions rather
>    than static megadriver_stub_init.
>  * Incorporate fixes and suggestions from Keith
>
> Signed-off-by: Jordan Justen <jordan.l.justen at intel.com>
> Cc: "10.0" <mesa-stable at lists.freedesktop.org>
> ---
>  src/mesa/drivers/dri/common/megadriver_stub.c | 125 ++++++++++++++++++++++++++
>  1 file changed, 125 insertions(+)
>
> diff --git a/src/mesa/drivers/dri/common/megadriver_stub.c b/src/mesa/drivers/dri/common/megadriver_stub.c
> index 6bf5d73..cb1078d 100644
> --- a/src/mesa/drivers/dri/common/megadriver_stub.c
> +++ b/src/mesa/drivers/dri/common/megadriver_stub.c
> @@ -23,6 +23,131 @@
>
>  #include <stdio.h>
>  #include "dri_util.h"
> +#include <dlfcn.h>
> +#include "main/macros.h"
> +
> +/* The extensions that allow the megadriver stub to provide backward
> + * compatibility for the older DRI driver loader require GNU
> + * extensions from dlfcn.h.
> + */
> +#ifdef _GNU_SOURCE

More specifically, what extension do we use?

> +#define MEGADRIVER_STUB_MAX_EXTENSIONS 10
> +#define LIB_PATH_SUFFIX "_dri.so"
> +#define LIB_PATH_SUFFIX_LENGTH (sizeof(LIB_PATH_SUFFIX)-1)
> +
> +/* This is the table of extensions that the loader will dlsym() for.
> + *
> + * Initially it is empty for the megadriver stub, but the library
> + * contructor may initialize it based on the name of the library that

constructor

> + * is being loaded.
> + */
> +PUBLIC const __DRIextension *
> +__driDriverExtensions[MEGADRIVER_STUB_MAX_EXTENSIONS] = {
> +   NULL
> +};
> +
> +/**
> + * This is a contructor function for the megadriver dynamic library.

constructor

> + *
> + * When the driver is dlopen'ed, this function will run. It will
> + * search for the name of the foo_dri.so file that was opened using
> + * the dladdr function.
> + *
> + * After finding foo's name, it will call __driDriverGetExtensions_foo
> + * and use the return to update __driDriverExtensions to enable
> + * compatibility with older DRI driver loaders.
> + */
> +__attribute__((constructor)) static void
> +megadriver_stub_init(void)
> +{
> +   Dl_info info;
> +   char *driver_name;
> +   size_t name_len;
> +   char *get_extensions_name;
> +   const __DRIextension **(*get_extensions)(void);
> +   const __DRIextension **extensions;
> +   int i;
> +
> +   /* Call dladdr on __driDriverExtensions. We are really
> +    * interested in the returned info.dli_fname so we can
> +    * figure out the path name of the library being loaded.
> +    */
> +   i = dladdr((void*) __driDriverExtensions, &info);
> +   if (i == 0)
> +      return;
> +
> +   /* Search for the last '/' character in the path. */
> +   driver_name = strrchr(info.dli_fname, '/');
> +   if (driver_name != NULL) {
> +      /* Skip '/' character */
> +      driver_name++;
> +   } else {
> +      /* Try using the start of the path */
> +      driver_name = (char*) info.dli_fname;
> +   }
> +
> +   /* Make sure the path ends with _dri.so */
> +   name_len = strlen(driver_name);
> +   i = name_len - LIB_PATH_SUFFIX_LENGTH;
> +   if (i < 0 || strcmp(driver_name + i, LIB_PATH_SUFFIX) != 0)
> +      return;
> +
> +   /* Duplicate the string so we can modify it.
> +    * So far we've been using info.dli_fname.
> +    */
> +   driver_name = strdup(driver_name);
> +   if (!driver_name)
> +      return;
> +
> +   /* The path ends with _dri.so. Chop this part of the
> +    * string off. Then we'll have the driver's final name.
> +    */
> +   driver_name[i] = '\0';
> +
> +   i = asprintf(&get_extensions_name, "%s_%s",
> +                __DRI_DRIVER_GET_EXTENSIONS, driver_name);
> +   free(driver_name);
> +   if (i == -1)
> +      return;
> +
> +   /* dlsym to get the driver's get extensions function. We
> +    * don't have the dlopen handle, so we have to use
> +    * RTLD_DEFAULT. It seems unlikely that the symbol will
> +    * be found in another library, but this isn't optimal.
> +    */
> +   get_extensions = dlsym(RTLD_DEFAULT, get_extensions_name);
> +   free(get_extensions_name);
> +   if (!get_extensions)
> +      return;
> +
> +   /* Use the newer DRI loader entrypoint to find extensions.
> +    * We will then expose these extensions via the older
> +    * __driDriverExtensions symbol.
> +    */
> +   extensions = get_extensions();
> +
> +   /* Copy the extensions into the __driDriverExtensions array
> +    * we declared.
> +    */
> +   for (i = 0; i < ARRAY_SIZE(__driDriverExtensions); i++) {
> +      __driDriverExtensions[i] = extensions[i];
> +      if (extensions[i] == NULL)
> +         break;
> +   }
> +
> +   /* If the driver had more extensions than we reserved, then
> +    * bail out.
> +    */
> +   if (i == ARRAY_SIZE(__driDriverExtensions)) {
> +      __driDriverExtensions[0] = NULL;
> +      fprintf(stderr, "Megadriver stub did not reserve enough extension "
> +              "slots.\n");
> +      return;
> +   }
> +}
> +
> +#endif // #ifdef _GNU_SOURCE

C++-style comment.

Very cool. Have a

Reviewed-by: Matt Turner <mattst88 at gmail.com>