Re: Concerns regarding e17bebd049 ("dump: Set correct vaddr for ELF dump")

[Stephen Brennan <stephen.s.brennan@oracle.com>]

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Stephen Brennan <stephen.s.brennan@oracle.com> writes:
> 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.
>
> 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?

And of course I sent the wrong copy of the file. Attached is the program
I intended to send (which properly handles endianness and sets the vaddr
as expected).


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#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;

	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 (endian() != hdr.e_ident[EI_DATA]) {
		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 = 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;
}