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, 6 insertions, 0 deletions

diff --git a/elf/dl-origin.c b/elf/dl-origin.c
index 9f6b921b01..812f5dbb28 100644
--- a/elf/dl-origin.c
+++ b/elf/dl-origin.c
@@ -47,3 +47,9 @@ _dl_get_origin (void)
 
   return result;
 }
+
+char *
+_dl_canonicalize (int fd)
+{
+  return NULL;
+}