[Bug threads/28014] gdb coredumps when remote+kgdbing a system that OOMs too hard

["simark at simark dot ca" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28014

--- Comment #5 from Simon Marchi <simark at simark dot ca> ---
It dug a little bit.  I checked why GDB couldn't see any shared library, I
think there's a little bug in the MIPS-specific code where GDB uses a
MIPS-specific auxv entry to locate the base of the runtime loader.  It doesn't
take into account the main executable's runtime address.

With the hack below, I can get a shared library list.  I then idenfitied that
the current PC was in libc, so I downloaded and extracted
libc6_2.31-12_mips64el.deb in my sysroot.  The backtrace becomes a bit better:

(gdb) bt
#0  0x0000004003148b4c in raise () from
/tmp/investigation/lib/mips64el-linux-gnuabi64/libc.so.6
#1  0x000000400312fa50 in abort () from
/tmp/investigation/lib/mips64el-linux-gnuabi64/libc.so.6
Backtrace stopped: frame did not save the PC

To get further, we would need libc's debug info.  I tried to find debug info
for libc.so.6 but can't find it.  It's supposed to be in the libc6 package,
which I think should be found here:

http://debug.mirrors.debian.org/debian-debug/pool/main/g/glibc/

But I can't find the debug package corresponding to libc6_2.31-12_mips64el.deb.
The libc's build-id is 59aa6ff984aff00883acda7feef7613cce475991, I can't find
that in any of the packages.  Do you have an idea?

>From a52228d3bb10cc6d1f6d748d22ee8a4cabff3304 Mon Sep 17 00:00:00 2001
From: Simon Marchi <simon.marchi@polymtl.ca>
Date: Sat, 26 Jun 2021 00:16:55 -0400
Subject: [PATCH] fix

Change-Id: I0d90e5432a5a998840dae7446bfcdb8995cf0297
---
 gdb/solib-svr4.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gdb/solib-svr4.c b/gdb/solib-svr4.c
index a8a7d1171dc6..d60427cd46bc 100644
--- a/gdb/solib-svr4.c
+++ b/gdb/solib-svr4.c
@@ -798,7 +798,7 @@ elf_locate_base (void)
       pbuf = (gdb_byte *) alloca (pbuf_size);
       /* DT_MIPS_RLD_MAP_REL contains an offset from the address of the
         DT slot to the address of the dynamic link structure.  */
-      if (target_read_memory (dyn_ptr + dyn_ptr_addr, pbuf, pbuf_size))
+      if (target_read_memory (dyn_ptr + dyn_ptr_addr + 0x4000000000, pbuf,
pbuf_size))
        return 0;
       return extract_typed_address (pbuf, ptr_type);
     }
-- 
2.32.0

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