[musl] [PATCH] ldso: support DT_RELR relative relocation format

[musl] [PATCH] ldso: support DT_RELR relative relocation format

[Fangrui Song]

this resolves DT_RELR relocations in non-ldso objects.

generic-abi pre-standard for DT_RELR:
https://groups.google.com/g/generic-abi/c/bX460iggiKg
FreeBSD rtld added DT_RELR support in 2021-08.
glibc added DT_RELR support in 2022-04.

Since ld.lld 7, --pack-dyn-relocs=relr can generate DT_RELR.
Since binutils 2.38, GNU ld's x86 and powerpc64 ports supports -z
pack-relative-relocs to generate DT_RELR. ld.lld 15 also has the option.

---
Changes from https://www.openwall.com/lists/musl/2019/03/06/3
* rename some variables
* decode_vec: add `if (v[0] < 8*sizeof(long))`
---
 include/elf.h          |  8 ++++++--
 ldso/dynlink.c         | 20 +++++++++++++++++++-
 src/internal/dynlink.h |  2 +-
 3 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/include/elf.h b/include/elf.h
index 86e2f0bb..9e980a29 100644
--- a/include/elf.h
+++ b/include/elf.h
@@ -385,7 +385,8 @@ typedef struct {
 #define SHT_PREINIT_ARRAY 16
 #define SHT_GROUP	  17
 #define SHT_SYMTAB_SHNDX  18
-#define	SHT_NUM		  19
+#define SHT_RELR	  19
+#define	SHT_NUM		  20
 #define SHT_LOOS	  0x60000000
 #define SHT_GNU_ATTRIBUTES 0x6ffffff5
 #define SHT_GNU_HASH	  0x6ffffff6
@@ -754,7 +755,10 @@ typedef struct {
 #define DT_PREINIT_ARRAY 32
 #define DT_PREINIT_ARRAYSZ 33
 #define DT_SYMTAB_SHNDX	34
-#define	DT_NUM		35
+#define DT_RELRSZ	35
+#define DT_RELR		36
+#define DT_RELRENT	37
+#define	DT_NUM		38
 #define DT_LOOS		0x6000000d
 #define DT_HIOS		0x6ffff000
 #define DT_LOPROC	0x70000000
diff --git a/ldso/dynlink.c b/ldso/dynlink.c
index 5b9c8be4..a50ef00a 100644
--- a/ldso/dynlink.c
+++ b/ldso/dynlink.c
@@ -208,7 +208,8 @@ static void decode_vec(size_t *v, size_t *a, size_t cnt)
 	size_t i;
 	for (i=0; i<cnt; i++) a[i] = 0;
 	for (; v[0]; v+=2) if (v[0]-1<cnt-1) {
-		a[0] |= 1UL<<v[0];
+		if (v[0] < 8*sizeof(long))
+			a[0] |= 1UL<<v[0];
 		a[v[0]] = v[1];
 	}
 }
@@ -513,6 +514,22 @@ static void do_relocs(struct dso *dso, size_t *rel, size_t rel_size, size_t stri
 	}
 }
 
+static void do_relr_relocs(struct dso *dso, size_t *relr, size_t relr_size) {
+	unsigned char *base = dso->base;
+	size_t *reloc_addr;
+	for (; relr_size; relr++, relr_size-=sizeof(size_t))
+		if ((relr[0]&1) == 0) {
+			reloc_addr = laddr(dso, relr[0]);
+			*reloc_addr++ += (size_t)base;
+		} else {
+			int i = 0;
+			for (size_t bitmap=relr[0]; (bitmap>>=1); i++)
+				if (bitmap&1)
+					reloc_addr[i] += (size_t)base;
+			reloc_addr += 8*sizeof(size_t)-1;
+		}
+}
+
 static void redo_lazy_relocs()
 {
 	struct dso *p = lazy_head, *next;
@@ -1355,6 +1372,7 @@ static void reloc_all(struct dso *p)
 			2+(dyn[DT_PLTREL]==DT_RELA));
 		do_relocs(p, laddr(p, dyn[DT_REL]), dyn[DT_RELSZ], 2);
 		do_relocs(p, laddr(p, dyn[DT_RELA]), dyn[DT_RELASZ], 3);
+		do_relr_relocs(p, laddr(p, dyn[DT_RELR]), dyn[DT_RELRSZ]);
 
 		if (head != &ldso && p->relro_start != p->relro_end &&
 		    mprotect(laddr(p, p->relro_start), p->relro_end-p->relro_start, PROT_READ)
diff --git a/src/internal/dynlink.h b/src/internal/dynlink.h
index 51c0639f..830354eb 100644
--- a/src/internal/dynlink.h
+++ b/src/internal/dynlink.h
@@ -93,7 +93,7 @@ struct fdpic_dummy_loadmap {
 #endif
 
 #define AUX_CNT 32
-#define DYN_CNT 32
+#define DYN_CNT 37
 
 typedef void (*stage2_func)(unsigned char *, size_t *);
 
-- 
2.36.1

[musl] Re: [PATCH] ldso: support DT_RELR relative relocation format

[Fangrui Song]

On 2022-06-05, Fangrui Song wrote:
>this resolves DT_RELR relocations in non-ldso objects.
>
>generic-abi pre-standard for DT_RELR:
>https://groups.google.com/g/generic-abi/c/bX460iggiKg
>FreeBSD rtld added DT_RELR support in 2021-08.
>glibc added DT_RELR support in 2022-04.
>
>Since ld.lld 7, --pack-dyn-relocs=relr can generate DT_RELR.
>Since binutils 2.38, GNU ld's x86 and powerpc64 ports supports -z
>pack-relative-relocs to generate DT_RELR. ld.lld 15 also has the option.

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