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Re: Concerns regarding e17bebd049 ("dump: Set correct vaddr for ELF dump
From: |
Laszlo Ersek |
Subject: |
Re: Concerns regarding e17bebd049 ("dump: Set correct vaddr for ELF dump") |
Date: |
Thu, 21 Sep 2023 09:49:00 +0200 |
On 9/20/23 19:35, Stephen Brennan wrote:
> Hi Jon,
>
> Jon Doron <arilou@gmail.com> writes:
>> Hi Stephen,
>> Like you have said the reason is as I wrote in the commit message,
>> without "fixing" the vaddr GDB is messing up mapping and working with
>> the generated core file.
>
> For the record I totally love this workaround :)
>
> It's clever and gets the job done and I would have done it in a
> heartbeat. It's just that it does end up making vmcores that have
> incorrect data, which is a pain for debuggers that are actually designed
> to look at kernel core dumps.
>
>> This patch is almost 4 years old, perhaps some changes to GDB has been
>> introduced to resolve this, I have not checked since then.
>
> Program Headers:
> Type Offset VirtAddr PhysAddr
> FileSiz MemSiz Flags Align
> NOTE 0x0000000000000168 0x0000000000000000 0x0000000000000000
> 0x0000000000001980 0x0000000000001980 0x0
> LOAD 0x0000000000001ae8 0x0000000000000000 0x0000000000000000
> 0x0000000080000000 0x0000000080000000 0x0
> LOAD 0x0000000080001ae8 0x0000000000000000 0x00000000fffc0000
> 0x0000000000040000 0x0000000000040000 0x0
>
> (gdb) info files
> Local core dump file:
> `/home/stepbren/repos/test_code/elf/dumpfile', file type elf64-x86-64.
> 0x0000000000000000 - 0x0000000080000000 is load1
> 0x0000000000000000 - 0x0000000000040000 is load2
>
> $ gdb --version
> GNU gdb (GDB) Red Hat Enterprise Linux 10.2-10.0.2.el9
> Copyright (C) 2021 Free Software Foundation, Inc.
> License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
> This is free software: you are free to change and redistribute it.
> There is NO WARRANTY, to the extent permitted by law.
>
>
> It doesn't *look like* anything has changed in this version of GDB. But
> I'm not really certain that GDB is expected to use the physical
> addresses in the load segments: it's not a kernel debugger.
The paging=off vmcores dumped by QEMU are primarily meant for the
"crash" utility <https://github.com/crash-utility/crash.git>, not gdb.
Crash builds upon gdb (it downloads a gdb tarball at build time, IIRC),
but either way, the vmcores are meant to be consumed by "crash", and
crash *is* a kernel debugger (both live, and post-mortem).
So, from my perspective: "whatever works with 'crash'". If you revert
Jon's commit and the vmcores continue working with "crash", I won't object.
I commented similarly under Jon's v1 patch -- as long as paging=off
dumps continue working with "crash", I'm fine:
http://mid.mail-archive.com/7961a154-f139-af73-613d-94b88bf95392@redhat.com
For reference, these are the v1 through v3 patch threads, from 2019:
http://mid.mail-archive.com/20181225125344.4482-1-arilou@gmail.com
http://mid.mail-archive.com/20190108130219.18550-1-arilou@gmail.com
http://mid.mail-archive.com/20190109082203.27142-1-arilou@gmail.com
Laszlo
>
> I think hacking the p_vaddr field _is_ the way to get GDB to behave in
> the way you want: allow you to read physical memory addresses.
>
>> As I'm no longer using this feature and have not worked and tested it
>> in a long while, so I have no obligations to this change, but perhaps
>> someone else might be using it...
>
> I definitely think it's valuable for people to continue being able to
> use QEMU vmcores generated with paging=off in GDB, even if GDB isn't
> desgined for it. It seems like a useful hack that appeals to the lowest
> common denominator: most people have GDB and not a purpose-built kernel
> debugger. But maybe we could point to a program like the below that will
> tweak the p_paddr field after the fact, in order to appeal to GDB's
> sensibilities?
>
> Thanks,
> Stephen
>
> ---
> #include <stdbool.h>
> #include <stdlib.h>
> #include <stdio.h>
> #include <string.h>
> #include <byteswap.h>
>
> #include <elf.h>
>
> static void fail(const char *msg)
> {
> fprintf(stderr, "%s\n", msg);
> exit(EXIT_FAILURE);
> }
>
> static void perror_fail(const char *pfx)
> {
> perror(pfx);
> exit(EXIT_FAILURE);
> }
>
> static void usage(void)
> {
> puts("usage: phys2virt COREFILE");
> puts("Modifies the ELF COREFILE so that load segments have their
> virtual");
> puts("address value copied from the physical address field.");
> exit(EXIT_SUCCESS);
> }
>
> static int endian(void)
> {
> union {
> uint32_t ival;
> char cval[4];
> } data;
> data.ival = 1;
> if (data.cval[0])
> return ELFDATA2LSB;
> else
> return ELFDATA2MSB;
> }
>
> int main(int argc, char **argv)
> {
> char *filename;
> FILE *f;
> Elf64_Ehdr hdr;
> Elf64_Phdr *phdrs;
> off_t phoff;
> int phnum, phentsize;
> bool bswap;
>
> if (argc != 2 || strcmp(argv[1], "-h") == 0)
> usage();
>
> filename = argv[1];
> f = fopen(filename, "r+");
> if (!f)
> perror_fail("open");
>
> if (fread(&hdr, sizeof(hdr), 1, f) != 1)
> perror_fail("read elf header");
>
> if (memcmp(hdr.e_ident, ELFMAG, 4) != 0)
> fail("not an ELF file");
>
> if (hdr.e_ident[EI_CLASS] != ELFCLASS64)
> fail("file is not 64-bits: unsupported");
>
> if (bswap) {
> phoff = bswap_64(hdr.e_phoff);
> phnum = bswap_16(hdr.e_phnum);
> phentsize = bswap_16(hdr.e_phentsize);
> } else {
> phoff = hdr.e_phoff;
> phnum = hdr.e_phnum;
> phentsize = hdr.e_phentsize;
> }
> if (phentsize != sizeof(Elf64_Phdr))
> fail("error: mismatch between phentsize and
> sizeof(Elf64_Phdr)");
>
> if (fseek(f, phoff, SEEK_SET) < 0)
> perror_fail("fseek");
>
> phdrs = calloc(phnum, phentsize);
> if (!phdrs)
> fail("error: allocation error");
>
> if (fread(phdrs, phentsize, phnum, f) != phnum)
> perror_fail("fread phdrs");
>
> for (int i = 0; i < phnum; i++)
> phdrs[i].p_vaddr = 0; //phdrs[i].p_paddr;
>
> if (fseek(f, phoff, SEEK_SET) < 0)
> perror_fail("fseek");
> if (fwrite(phdrs, phentsize, phnum, f) != phnum)
> perror_fail("fwrite phdrs");
>
> fclose(f);
> return EXIT_SUCCESS;
> }
>