Alan Davis discovered a problem using shared libraries with textrel symbols on nommu (C6X). He writes:
"I think I've found a bug in the dynamic loader's dynamic DSBT index assignment. When I try to execute a program that has a textrel DSO, it fails to load. The telltale line in the LD_DEBUG output is: _dl_get_ready_to_run:779: file=''; needed by './a.out' The corresponding DT_NEEDED entry has 'libc.so.0', but here the filename is empty. This is what is happening in _dl_elf_shared_library(): First, map all segments according to their permissions. Text gets initially mapped read-only. Then, parse the dynamic information. The dynamic table is in RW but some of the tags may point to RO. For example, DT_NEEDED points to a string in .dynstr which is in RO. These pointers get computed according to the loadmap from the original mapping. Then, in response to a DT_TEXTREL tag, the RO segment gets remapped, thereby invaliding anything that points to it, in particular certain dynamic tags such as DT_NEEDED. I validated this by applying the following patch, which re-parses the dynamic info after the remapping so as to re-compute any invalid pointers. There are perhaps better solutions; perhaps split the tag parsing into two passes, the first of which scans only for DT_TEXTREL and then the rest can happen after remapping." I've looked over the patch and it looks good to me. Ok to apply? Bernd
>From 585996caa7466119716ce1f477a52bc42015946b Mon Sep 17 00:00:00 2001 From: Alan Davis <[email protected]> Date: Thu, 26 May 2011 18:15:06 +0200 Subject: [PATCH] Correct a bug when remapping textrel segments on nommu From: Alan Davis <[email protected]> On C6X, when trying to execute a program that has a textrel DSO, it fails to load. The telltale line in the LD_DEBUG output is: _dl_get_ready_to_run:779: file=''; needed by './a.out' The corresponding DT_NEEDED entry has 'libc.so.0', but here the filename is empty. This is what is happening in _dl_elf_shared_library(): First, map all segments according to their permissions. Text gets initially mapped read-only. Then, parse the dynamic information. The dynamic table is in RW but some of the tags may point to RO. For example, DT_NEEDED points to a string in .dynstr which is in RO. These pointers get computed according to the loadmap from the original mapping. Then, in response to a DT_TEXTREL tag, the RO segment gets remapped, thereby invaliding anything that points to it, in particular certain dynamic tags such as DT_NEEDED. The following patch re-parses the dynamic info after the remapping so as to re-compute any invalid pointers. Signed-off-by: Bernd Schmidt <[email protected]> --- ldso/ldso/dl-elf.c | 2 ++ 1 files changed, 2 insertions(+), 0 deletions(-) diff --git a/ldso/ldso/dl-elf.c b/ldso/ldso/dl-elf.c index 7b5d751..2352fcd 100644 --- a/ldso/ldso/dl-elf.c +++ b/ldso/ldso/dl-elf.c @@ -714,6 +714,8 @@ struct elf_resolve *_dl_load_elf_shared_library(int secure, DL_UPDATE_LOADADDR_HDR(lib_loadaddr, new_addr + (ppnt->p_vaddr & ADDR_ALIGN), ppnt); + /* Update any pointers into remapped segments. */ + _dl_parse_dynamic_info(dpnt, dynamic_info, NULL, lib_loadaddr); #endif } } -- 1.7.3.4
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