PR/59296 CVS commit: src

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

The following reply was made to PR port-mips/59296; it has been noted by GNATS.

From: "Taylor R Campbell" <riastradh%netbsd.org@localhost>
To: gnats-bugs%gnats.NetBSD.org@localhost
Cc: 
Subject: PR/59296 CVS commit: src
Date: Wed, 16 Apr 2025 01:56:53 +0000

 Module Name:	src
 Committed By:	riastradh
 Date:		Wed Apr 16 01:56:53 UTC 2025
 
 Modified Files:
 	src/libexec/ld.elf_so: headers.c
 	src/sys/arch/mips/include: elf_machdep.h
 	src/tests/libexec/ld.elf_so: t_rtld_r_debug.c
 
 Log Message:
 ld.elf_so: Teach this to handle MIPS PIE rtld debug data.
 
 Adapt t_rtld_r_debug to handle the two MIPS cases too.
 
 XXX t_rtld_r_debug should be tested both as PIE and non-PIE to
 exercise both cases.
 
 Context:
 
 The value of a DT_DEBUG .dynamic entry is initialized at load-time,
 by ld.elf_so, to a pointer to a data structure set up by ld.elf_so
 describing the shared objects loaded by the executable, so debuggers
 can find them from, e.g., core dumps.  None of this is really
 documented anywhere that I can find.  Best reference is this post on
 the gdb mailing list from a quarter century ago saying there's no
 real documentation:
 
 https://web.archive.org/web/20250414021320/https://sourceware.org/pipermail/gdb/2000-April/004509.html
 
 However, on MIPS, the .dynamic section is mapped read-only, so
 ld.elf_so can't properly modify it (I imagine technically it could
 with some mprotect shenanigans but that's not how it's done on MIPS).
 Instead, the linker reserves a location in read/write memory and uses
 a DT_MIPS_RLD_MAP entry with a pointer to that location.
 
 However, in position-independent executables, the .dynamic entry
 can't have an absolute pointer to that location because it's not
 known up front.  Instead, the the linker uses a DT_MIPS_RLD_MAP_REL
 entry with the relative offset to that location from the Elf_Dyn
 entry itself.
 
 I would add a reference for this but it's basically a matter of UTSL
 plus some oblique mentions on the web and mailing list discussions:
 
 https://web.archive.org/web/20250414024823/https://cygwin.com/legacy-ml/binutils/2016-04/msg00244.html
 https://web.archive.org/web/20250403151803/https://maskray.me/blog/2023-09-04-toolchain-notes-on-mips
 https://web.archive.org/web/20211024050833/https://reviews.llvm.org/D12794?id=34533
 https://web.archive.org/web/20250407052145/https://wiki.debian.org/MIPSPort
 https://web.archive.org/web/20250414024924/https://reviews.freebsd.org/D17867?id=50122
 
 PR port-mips/59296: t_rtld_r_debug test is failing
 
 
 To generate a diff of this commit:
 cvs rdiff -u -r1.72 -r1.73 src/libexec/ld.elf_so/headers.c
 cvs rdiff -u -r1.20 -r1.21 src/sys/arch/mips/include/elf_machdep.h
 cvs rdiff -u -r1.10 -r1.11 src/tests/libexec/ld.elf_so/t_rtld_r_debug.c
 
 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.