Stephen Brennan <stephen.s.bren...@oracle.com> writes:
> Hi Jon,
>
> Jon Doron <ari...@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).
#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;
}
