[PATCH v4 2/5] elf: Properly handle zero DT_RELA/DT_REL values

[Fangrui Song]

On 2022-03-01, H.J. Lu wrote:
>With DT_RELR, there may be no relocations in DT_RELA/DT_REL and their
>entry values are zero.  Don't relocate DT_RELA/DT_REL and update the
>combined relocation start address if their entry values are zero.
>---
> elf/dynamic-link.h     |  6 +++++-
> elf/get-dynamic-info.h | 18 ++++++++++++++----
> 2 files changed, 19 insertions(+), 5 deletions(-)
>
>diff --git a/elf/dynamic-link.h b/elf/dynamic-link.h
>index 5318079bac..320a9029f2 100644
>--- a/elf/dynamic-link.h
>+++ b/elf/dynamic-link.h
>@@ -84,7 +84,9 @@ elf_machine_lazy_rel (struct link_map *map, struct r_scope_elem *scope[],
> 	     __typeof (((ElfW(Dyn) *) 0)->d_un.d_val) nrelative; int lazy; }  \
>       ranges[2] = { { 0, 0, 0, 0 }, { 0, 0, 0, 0 } };			      \
> 									      \
>-    if ((map)->l_info[DT_##RELOC])					      \
>+    /* With DT_RELR, DT_RELA/DT_REL can have zero value.  */		      \
>+    if ((map)->l_info[DT_##RELOC]					      \
>+	&& (map)->l_info[DT_##RELOC]->d_un.d_ptr != 0)			      \
>       {									      \
> 	ranges[0].start = D_PTR ((map), l_info[DT_##RELOC]);		      \
> 	ranges[0].size = (map)->l_info[DT_##RELOC##SZ]->d_un.d_val;	      \
>@@ -98,6 +100,8 @@ elf_machine_lazy_rel (struct link_map *map, struct r_scope_elem *scope[],
> 	ElfW(Addr) start = D_PTR ((map), l_info[DT_JMPREL]);		      \
> 	ElfW(Addr) size = (map)->l_info[DT_PLTRELSZ]->d_un.d_val;	      \
> 									      \
>+	if (ranges[0].start == 0)					      \
>+	  ranges[0].start = start;					      \
> 	if (ranges[0].start + ranges[0].size == (start + size))		      \
> 	  ranges[0].size -= size;					      \
> 	if (!(do_lazy)							      \
>diff --git a/elf/get-dynamic-info.h b/elf/get-dynamic-info.h
>index 6c2a3a12b1..f4b957684b 100644
>--- a/elf/get-dynamic-info.h
>+++ b/elf/get-dynamic-info.h
>@@ -83,16 +83,26 @@ elf_get_dynamic_info (struct link_map *l, bool bootstrap,
>       ADJUST_DYN_INFO (DT_PLTGOT);
>       ADJUST_DYN_INFO (DT_STRTAB);
>       ADJUST_DYN_INFO (DT_SYMTAB);
>+      ADJUST_DYN_INFO (DT_RELR);
>+      ADJUST_DYN_INFO (DT_JMPREL);
>+      ADJUST_DYN_INFO (VERSYMIDX (DT_VERSYM));
>+      ADJUST_DYN_INFO (ADDRIDX (DT_GNU_HASH));
>+# undef ADJUST_DYN_INFO
>+
>+      /* DT_RELA/DT_REL are mandatory.  But they may have zero value if
>+	 there is DT_RELR.  Don't relocate them if they are zero.  */
>+# define ADJUST_DYN_INFO(tag) \
>+      do								      \
>+	if (info[tag] != NULL && info[tag]->d_un.d_ptr != 0)		      \
>+         info[tag]->d_un.d_ptr += l_addr;				      \
>+      while (0)
>+
> # if ! ELF_MACHINE_NO_RELA
>       ADJUST_DYN_INFO (DT_RELA);
> # endif
> # if ! ELF_MACHINE_NO_REL
>       ADJUST_DYN_INFO (DT_REL);
> # endif
>-      ADJUST_DYN_INFO (DT_RELR);
>-      ADJUST_DYN_INFO (DT_JMPREL);
>-      ADJUST_DYN_INFO (VERSYMIDX (DT_VERSYM));
>-      ADJUST_DYN_INFO (ADDRIDX (DT_GNU_HASH));
> # undef ADJUST_DYN_INFO
>     }
>   if (info[DT_PLTREL] != NULL)
>-- 
>2.35.1
>

Reviewed-by: Fangrui Song <maskray@google.com>