[PATCH] ARC: Support R_ARC_JMP_SLOT relocations during bootstrap

Andrew Burgess andrew.burgess at embecosm.com
Thu Jul 28 17:59:08 UTC 2016

I ran into this issue while trying to debug a _different_ issue witin
the uClinc(-ng) arc dynamic linker.  I turned on debugging support
within the dynamic linker, and the linker would no longer complete its
bootstrap phase due to a lack of support for the R_ARC_JMP_SLOT
relocation type.

To reproduce this issue it should be enough to configure uClibc(-ng)
for ARC with 'DODEBUG=y', then try to run anything that requires
dynamic linking.

The R_ARC_JMP_SLOT relocation type is used within the .plt, so I
believe it makes sense for these relocations to be generated.

I updated the associated comment above the boostrap patching code so
that it makes more sense (to me at least) with the extra relocation

I wonder if you would consider merging this patch?



This commit adds support for R_ARC_JMP_SLOT relocations during the
bootstrap phase of the dynamic linker.  These relocations will be
generated if uClibc is configured with 'DODEBUG=y'.
 ldso/ldso/arc/dl-startup.h | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/ldso/ldso/arc/dl-startup.h b/ldso/ldso/arc/dl-startup.h
index ef89b53..fadc433 100644
--- a/ldso/ldso/arc/dl-startup.h
+++ b/ldso/ldso/arc/dl-startup.h
@@ -64,10 +64,11 @@ __asm__(
  * Dynamic loader bootstrapping:
- * Since we don't modify text at runtime, these can only be data relos
- * (so safe to assume that they are word aligned).
- * And also they HAVE to be RELATIVE relos only
- * @RELP is the relo entry being processed
+ * The only relocations that should be found are either R_ARC_RELATIVE for
+ * data relocations (.got, etc) or R_ARC_JMP_SLOT for code relocations
+ * (.plt).  It is safe to assume that all of these relocations are word
+ * aligned.
+ * @RELP is the reloc entry being processed
  * @REL is the pointer to the address we are relocating.
  * @SYMBOL is the symbol involved in the relocation
  * @LOAD is the load address.
@@ -78,6 +79,8 @@ do {									\
 	int type = ELF32_R_TYPE((RELP)->r_info);			\
 	if (likely(type == R_ARC_RELATIVE))				\
 		*REL += (unsigned long) LOAD;				\
+	else if (type == R_ARC_JMP_SLOT)                                \
+		*REL = SYMBOL;						\
 	else								\
 		_dl_exit(1);						\

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