[PATCH] mprotect segments with extra PROT_WRITE bit when DT_TEXTREL bit is set

[Fangrui Song]

On 2018-08-22, Fangrui Song wrote:
>Currently, DT_TEXTREL is incompatible with IFUNC. When DT_TEXTREL or
>DF_TEXTREL is seen, the dynamic linker calls __mprotect on the segments
>with PROT_READ|PROT_WRITE before applying dynamic relocations. It leads
>to segfault when performing IFUNC resolution.
>
>This patch makes it call __mprotect with extra PROT_WRITE bit, which
>will keep the PROT_EXEC bit if exists, and thus fixes the segfault.
>FreeBSD rtld libexec/rtld-elf/rtld.c (reloc_textrel_prot) does the same.
>
>2018-08-22  Fangrui Song  <maskray@google.com>
>
>	* elf/dl-reloc.c (_dl_relocate_object): __mprotect with extra
>	PROT_WRITE bit.
>
>diff --git a/elf/dl-reloc.c b/elf/dl-reloc.c
>index 053916eeae..bd7d1ae84f 100644
>--- a/elf/dl-reloc.c
>+++ b/elf/dl-reloc.c
>@@ -199,14 +199,6 @@ _dl_relocate_object (struct link_map *l, struct r_scope_elem *scope[],
>			- ALIGN_DOWN (ph->p_vaddr, GLRO(dl_pagesize));
>	    newp->start = PTR_ALIGN_DOWN (ph->p_vaddr, GLRO(dl_pagesize))
>			  + (caddr_t) l->l_addr;
>-
>-	    if (__mprotect (newp->start, newp->len, PROT_READ|PROT_WRITE) < 0)
>-	      {
>-		errstring = N_("cannot make segment writable for relocation");
>-	      call_error:
>-		_dl_signal_error (errno, l->l_name, NULL, errstring);
>-	      }
>-
>#if (PF_R | PF_W | PF_X) == 7 && (PROT_READ | PROT_WRITE | PROT_EXEC) == 7
>	    newp->prot = (PF_TO_PROT
>			  >> ((ph->p_flags & (PF_R | PF_W | PF_X)) * 4)) & 0xf;
>@@ -220,6 +212,14 @@ _dl_relocate_object (struct link_map *l, struct r_scope_elem *scope[],
>	      newp->prot |= PROT_EXEC;
>#endif
>	    newp->next = textrels;
>+
>+	    if (__mprotect (newp->start, newp->len, newp->prot|PROT_WRITE) < 0)
>+	      {
>+		errstring = N_("cannot make segment writable for relocation");
>+	      call_error:
>+		_dl_signal_error (errno, l->l_name, NULL, errstring);
>+	      }
>+
>	    textrels = newp;
>	  }
>    }

A simple reproduce.

// a.c
void f_imp(void) {}
void (*resolve_f(void))(void) { return f_imp; }
void f(void) __attribute__((ifunc("resolve_f")));
int main(void) { f(); }

% clang -fuse-ld=lld -Wl,-z,notext a.c -o a; ./a
[1]    88059 segmentation fault  ./a
% clang -fuse-ld=lld -Wl,-z,notext a.c -Wl,--dynamic-linker=~/Dev/Util/glibc/build/elf/ld.so -o a; ./a
# good

gcc is similar. But upstream gcc does not support -fuse-ld=lld. It is available
on Debian and its derivatives via a patch.

This works fine on FreeBSD because its rtld does the same as the patch does.
OpenBSD rtld is probably also faulty.


-- 
宋方睿