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rtld: Support DT_RELR relative relocation format

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PIE and shared objects usually have many relative relocations. In
2017/2018, a compact relative relocation format RELR was proposed on
https://groups.google.com/g/generic-abi/c/bX460iggiKg/m/GxjM0L-PBAAJ
("Proposal for a new section type SHT_RELR") and is a pre-standard.
RELR usually takes 3% or smaller space than R_*_RELATIVE relocations.
The virtual memory size of a mostly statically linked PIE is typically
5~10% smaller.

ld.lld --pack-dyn-relocs=relr emits RELR relocations. DT_RELR has been
adopted by Android bionic, Linux kernel's arm64 port, Chrome OS (patched
glibc).

This patch adds DT_RELR support to FreeBSD rtld-elf.

MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D32524
This commit is contained in:
Fangrui Song 2021-10-16 14:34:37 -07:00 committed by Konstantin Belousov
parent 1cf0633316
commit a7d137fcbc
2 changed files with 38 additions and 0 deletions
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36
libexec/rtld-elf/rtld.c
View file

@ -1255,6 +1255,18 @@ digest_dynamic1(Obj_Entry *obj, int early, const Elf_Dyn **dyn_rpath,
assert(dynp->d_un.d_val == sizeof(Elf_Rela));
break;
case DT_RELR:
obj->relr = (const Elf_Relr *)(obj->relocbase + dynp->d_un.d_ptr);
break;
case DT_RELRSZ:
obj->relrsize = dynp->d_un.d_val;
break;
case DT_RELRENT:
assert(dynp->d_un.d_val == sizeof(Elf_Relr));
break;
case DT_PLTREL:
plttype = dynp->d_un.d_val;
assert(dynp->d_un.d_val == DT_REL || plttype == DT_RELA);
@ -3148,6 +3160,29 @@ reloc_textrel_prot(Obj_Entry *obj, bool before)
return (0);
}
/* Process RELR relative relocations. */
static void
reloc_relr(Obj_Entry *obj)
{
const Elf_Relr *relr, *relrlim;
Elf_Addr *where;
relrlim = (const Elf_Relr *)((const char *)obj->relr + obj->relrsize);
for (relr = obj->relr; relr < relrlim; relr++) {
Elf_Relr entry = *relr;
if ((entry & 1) == 0) {
where = (Elf_Addr *)(obj->relocbase + entry);
*where++ += (Elf_Addr)obj->relocbase;
} else {
for (long i = 0; (entry >>= 1) != 0; i++)
if ((entry & 1) != 0)
where[i] += (Elf_Addr)obj->relocbase;
where += CHAR_BIT * sizeof(Elf_Relr) - 1;
}
}
}
/*
* Relocate single object.
* Returns 0 on success, or -1 on failure.
@ -3174,6 +3209,7 @@ relocate_object(Obj_Entry *obj, bool bind_now, Obj_Entry *rtldobj,
/* Process the non-PLT non-IFUNC relocations. */
if (reloc_non_plt(obj, rtldobj, flags, lockstate))
return (-1);
reloc_relr(obj);
/* Re-protected the text segment. */
if (obj->textrel && reloc_textrel_prot(obj, false) != 0)

2
libexec/rtld-elf/rtld.h
View file

@ -174,6 +174,8 @@ typedef struct Struct_Obj_Entry {
unsigned long relsize; /* Size in bytes of relocation info */
const Elf_Rela *rela; /* Relocation entries with addend */
unsigned long relasize; /* Size in bytes of addend relocation info */
const Elf_Relr *relr; /* RELR relocation entries */
unsigned long relrsize; /* Size in bytes of RELR relocations */
const Elf_Rel *pltrel; /* PLT relocation entries */
unsigned long pltrelsize; /* Size in bytes of PLT relocation info */
const Elf_Rela *pltrela; /* PLT relocation entries with addend */