Dear uclibc-ng devs, The LLVM linker seems to be quite clever. When resolving relocations, accesses to the GOT are potentially replaced by PC relative addressing to the requested symbol. This breaks the old method of calculating the load address by using an unrelocated GOT entry value. Instead, rely on __ehdr_start having a link address of zero.The LLVM linker seems to be quite clever. When resolving relocations, accesses to the GOT are potentially replaced by PC relative addressing to the requested symbol. This breaks the old method of calculating the load address by using an unrelocated GOT entry value. Instead, rely on __ehdr_start having a link address of zero.
The patch is attached. Best regards, - Marcus -- +++++++++++++++++++ Register now for our workshop "Get to know L4Re in 3 days" on April 8-10. Learn to design and deploy secure system architectures for your product with L4Re: https://www.kernkonzept.com/workshop-getting-started-with-l4re/ +++++++++++++++++++ Kernkonzept GmbH Sitz: Dresden HRB 31129 Geschäftsführer: Dr.-Ing. Michael Hohmuth
From 2df8a64bb5bb31e9325f0d9157d68eddcd208d40 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Jan=20Kl=C3=B6tzke?= <jan.kloet...@kernkonzept.com> Date: Wed, 19 Mar 2025 08:03:12 +0100 Subject: [PATCH] Do not rely on unrelocated GOT entries The LLVM linker seems to be quite clever. When resolving relocations, accesses to the GOT are potentially replaced by PC relative addressing to the requested symbol. This breaks the old method of calculating the load address by using an unrelocated GOT entry value. Instead, rely on __ehdr_start having a link address of zero. Signed-off-by: Marcus Haehnel <marcus.haeh...@kernkonzept.com> --- libc/sysdeps/linux/arm/crt1.S | 15 ++++++--------- libc/sysdeps/linux/x86_64/crt1.S | 6 +----- 2 files changed, 7 insertions(+), 14 deletions(-) diff --git a/libc/sysdeps/linux/arm/crt1.S b/libc/sysdeps/linux/arm/crt1.S index 799f11080..040ddfd27 100644 --- a/libc/sysdeps/linux/arm/crt1.S +++ b/libc/sysdeps/linux/arm/crt1.S @@ -248,13 +248,9 @@ _start: #if defined(__ARCH_USE_MMU__) || defined(__UCLIBC_FORMAT_ELF__) #ifdef L_rcrt1 /* We don't need to save a1 since no dynamic linker should have run */ - ldr a1, .L_GOT /* Get value at .L_GOT + 0 (offset to GOT)*/ - adr a2, .L_GOT /* Get address of .L_GOT */ - ldr a3, .L_GOT+16 /* Get value of _start(GOT) stored in .L_GOT */ - adr a4, _start /* Get address of _start after relocation (changes to pc - ~30 or so) */ - add a1, a1, a2 /* Calculate where the GOT is */ - ldr a2, [a1, a3] /* GOT + _start(GOT) = offset of _start from begin of file */ - sub a1, a4, a2 /* Current addr of _start - offset from beginning of file = load addr */ + adr a1, .L__ehdr_start_off /* Get address of .L__ehdr_start_off */ + ldr a2, .L__ehdr_start_off /* Offset from .L__ehdr_start_off to __ehdr_start */ + add a1, a1, a2 /* Address of __ehdr_start = load addr */ bl reloc_static_pie mov a1, #0 /* Clean up a1 so that a random address won't get called at the end of program */ @@ -325,9 +321,10 @@ _start: .word _fini(GOT) .word _init(GOT) .word main(GOT) -#ifdef L_rcrt1 - .word _start(GOT) #endif +#ifdef L_rcrt1 +.L__ehdr_start_off: + .word __ehdr_start - .L__ehdr_start_off #endif #endif diff --git a/libc/sysdeps/linux/x86_64/crt1.S b/libc/sysdeps/linux/x86_64/crt1.S index 701cbf2f6..151aeffeb 100644 --- a/libc/sysdeps/linux/x86_64/crt1.S +++ b/libc/sysdeps/linux/x86_64/crt1.S @@ -83,11 +83,7 @@ _start: #ifdef L_rcrt1 pushq %rdi /* save rdi (but should be 0...) */ pushq %rdx /* store rdx (rtld_fini) */ - xorq %rcx, %rcx /* ensure rcx is 0 */ - addq _start@GOTPCREL(%rip), %rcx /* get offset of _start from beginning of file */ - movq _start@GOTPCREL(%rip), %rax /* get run time address of _start */ - subq %rcx, %rax /* calculate run time load offset */ - movq %rax, %rdi /* load offset -> param 1 */ + lea __ehdr_start(%rip), %rdi /* "Calculate" load address... */ call reloc_static_pie /* relocate dynamic addrs */ xorq %rax, %rax /* cleanup */ popq %rdx -- 2.47.1
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