On Tue, 28 Dec 2010, Andrew Hastings wrote: > libhugetlbfs redefines shmget() so that it can transparently optionally > add the hugepage flag. Unfortunately, it uses dlsym() to to find the > next shmget() to call in the dynamic library call chain. This does not > work on statically-linked executables. > > Work around this by making all references to libdl weak, and testing > for its presence. If libdl symbols are not not present, directly > make the shmget syscall. > > Making the dl* symbols weak does not affect dynamic executables as > libdl.so is: > * Automatically added to the library dependency list when liked with > -lhugetlbfs > * Mapped into the programs address space when used with LD_PRELOAD. > > Based on a patch originally authored by Dean Luick <lu...@cray.com>. > > Signed-off-by: Andrew Hastings <a...@cray.com> on behalf of Cray Inc. > --- > > -Andrew Hastings > Cray Inc. > > diff -ruNp libhugetlbfs-2.11/shm.c libhugetlbfs-2.11-shm/shm.c > --- libhugetlbfs-2.11/shm.c 2010-12-16 11:38:22.000000000 -0600 > +++ libhugetlbfs-2.11-shm/shm.c 2010-12-28 17:26:00.725063000 -0600 > @@ -29,6 +29,37 @@ > #include "libhugetlbfs_internal.h" > #include "hugetlbfs.h" > > +#if defined(__x86_64)
Why is this patch x86_64 specific, does it not work for x86, or ppc64, or any
other arch we support? I am reluctant to take functionality like this for
a single arch unless there is no way to make it work elsewhere.
> +/*
> + * The calls to dlsym() and dlerror() in the shmget() wrapper below force
> + * a dependency on libdl.so. This does not work for static executables
> + * as the glibc dynamic library implementation does not automatically
> + * have static dl* function stubs linked into static executables.
> + *
> + * Work around this problem by adding a weak attribute to the declarations
> + * of dlsym() and dlerror(). (The declaration is otherwise the same as in
> + * <dlfcn.h>). This allows a static executable to be linked without -ldl.
> + * If &dlsym is NULL then this is a static executable and a call to the
> + * system shmget() may be performed without worry as there is no dynamic
> + * call chain.
> + *
> + * Although this problem appears to be platform-independent, the solution
> + * unfortunately depends on the platform, thus the __x86_64 ifdef.
> + */
> +#include <sys/syscall.h> /* for SYS_xxx definitions */
> +
> +extern void *dlsym (void *__restrict __handle, __const char *__restrict
> __name)
> + __attribute__((weak)) __THROW __nonnull ((2));
> +extern char *dlerror (void) __attribute__((weak)) __THROW;
> +
> +
> +/* call syscall shmget through the generic syscall mechanism */
> +static int syscall_shmget(key_t key, size_t size, int shmflg)
> +{
> + return syscall(SYS_shmget, key, size, shmflg);
> +}
> +#endif /* __x86_64 */
> +
> int shmget(key_t key, size_t size, int shmflg)
> {
> static int (*real_shmget)(key_t key, size_t size, int shmflg) = NULL;
> @@ -40,10 +71,18 @@ int shmget(key_t key, size_t size, int s
>
> /* Get a handle to the "real" shmget system call */
> if (!real_shmget) {
> - real_shmget = dlsym(RTLD_NEXT, "shmget");
> - if ((error = dlerror()) != NULL) {
> - ERROR("%s", error);
> - return -1;
> +#if defined(__x86_64)
> + if (&dlsym == NULL) {
> + /* in a static executable, call shmget directly */
> + real_shmget = syscall_shmget;
> + } else
> +#endif /* __x86_64 */
> + {
> + real_shmget = dlsym(RTLD_NEXT, "shmget");
> + if ((error = dlerror()) != NULL) {
> + ERROR("%s", error);
> + return -1;
> + }
> }
> }
>
>
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