elf: Canonicalize $ORIGIN in an explicit ld.so invocation [BZ 25263]azanella/bz25263-loader-realpath

When an executable is invoked directly, we calculate $ORIGIN by calling
readlink on /proc/self/exe, which the Linux kernel resolves to the
target of any symlinks.  However, if an executable is run through ld.so,
we cannot use /proc/self/exe and instead use the path given as an
argument. This leads to a different calculation of $ORIGIN, which is
most notable in that it causes ldd to behave differently (e.g., by not
finding a library) from directly running the program.

To make the behavior consistent, take advantage of the fact that the
kernel also resolves /proc/self/fd/ symlinks to the target of any
symlinks in the same manner, so once we have opened the main executable
in order to load it, replace the user-provided path with the result of
calling readlink("/proc/self/fd/N").

(On non-Linux platforms this resolution does not happen and so no
behavior change is needed.)

Co-authored-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index 8465cbaa9b..19494b82ee 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -1223,6 +1223,10 @@ extern struct link_map * _dl_get_dl_main_map (void) attribute_hidden;
 /* Find origin of the executable.  */
 extern const char *_dl_get_origin (void) attribute_hidden;
 
+/* Return the canonalized path name from the opened file descriptor FD,
+   or NULL otherwise.  */
+extern char * _dl_canonicalize (int fd) attribute_hidden;
+
 /* Count DSTs.  */
 extern size_t _dl_dst_count (const char *name) attribute_hidden;