* why ptrace read failed to read debugging process memory?
@ 2022-03-10 6:40 周春明(日月)
2022-03-10 10:05 ` Pedro Alves
0 siblings, 1 reply; 3+ messages in thread
From: 周春明(日月) @ 2022-03-10 6:40 UTC (permalink / raw)
To: 周春明(日月),
Simon Marchi, Gdb-patches, gdb-patches
Cc: Louis-He, Dominique Quatravaux, Sam Warner
Hi GDB maintainers,
I tried update our gdb10 to gdb12, but I found new gdb seems cannot pread debugging process memory.
3897 linux_proc_xfer_memory_partial (gdb_byte *readbuf, const gdb_byte *writebuf,
3898 ULONGEST offset, LONGEST len,
3899 ULONGEST *xfered_len)
3900 {
3901 ssize_t ret;
3902 auto iter = proc_mem_file_map.find (inferior_ptid.pid ());
3903 if (iter == proc_mem_file_map.end ())
3904 return TARGET_XFER_EOF;
3905
3906 int fd = iter->second.fd ();
3907
3908 gdb_assert (fd != -1);
3909
3910 /* Use pread64/pwrite64 if available, since they save a syscall and can
3911 handle 64-bit offsets even on 32-bit platforms (for instance, SPARC
3912 debugging a SPARC64 application). */
3913 #ifdef HAVE_PREAD64
3914 ret = (readbuf ? pread64 (fd, readbuf, len, offset)
3915 : pwrite64 (fd, writebuf, len, offset));
3916 #else
3917 ret = lseek (fd, offset, SEEK_SET);
3918 if (ret != -1)
3919 ret = (readbuf ? read (fd, readbuf, len)
3920 : write (fd, writebuf, len));
3921 #endif
3922
3923 if (ret == -1)
3924 {
3925 printf ("accessing fd %d for pid %d failed: %s (%d)\n", ================> here always returns -EIO (5) errno.
3926 fd, inferior_ptid.pid (),
3927 safe_strerror (errno), errno);
3928 return TARGET_XFER_EOF;
3929 }
any configure I missed in new GDB12? or new ptrace way needed?
Thanks very much,
-David
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: why ptrace read failed to read debugging process memory?
2022-03-10 6:40 why ptrace read failed to read debugging process memory? 周春明(日月)
@ 2022-03-10 10:05 ` Pedro Alves
2022-03-10 10:34 ` 回复:why " 周春明(日月)
0 siblings, 1 reply; 3+ messages in thread
From: Pedro Alves @ 2022-03-10 10:05 UTC (permalink / raw)
To: 周春明(日月),
Simon Marchi, Gdb-patches, gdb-patches
Cc: Louis-He, Dominique Quatravaux, Sam Warner
On 2022-03-10 06:40, 周春明(日月) via Gdb-patches wrote:
> Hi GDB maintainers,
> I tried update our gdb10 to gdb12, but I found new gdb seems cannot pread debugging process memory.
>
> 3897 linux_proc_xfer_memory_partial (gdb_byte *readbuf, const gdb_byte *writebuf,
> 3898 ULONGEST offset, LONGEST len,
> 3899 ULONGEST *xfered_len)
> 3900 {
> 3901 ssize_t ret;
> 3902 auto iter = proc_mem_file_map.find (inferior_ptid.pid ());
> 3903 if (iter == proc_mem_file_map.end ())
> 3904 return TARGET_XFER_EOF;
> 3905
> 3906 int fd = iter->second.fd ();
> 3907
> 3908 gdb_assert (fd != -1);
> 3909
> 3910 /* Use pread64/pwrite64 if available, since they save a syscall and can
> 3911 handle 64-bit offsets even on 32-bit platforms (for instance, SPARC
> 3912 debugging a SPARC64 application). */
> 3913 #ifdef HAVE_PREAD64
> 3914 ret = (readbuf ? pread64 (fd, readbuf, len, offset)
> 3915 : pwrite64 (fd, writebuf, len, offset));
> 3916 #else
> 3917 ret = lseek (fd, offset, SEEK_SET);
> 3918 if (ret != -1)
> 3919 ret = (readbuf ? read (fd, readbuf, len)
> 3920 : write (fd, writebuf, len));
> 3921 #endif
> 3922
> 3923 if (ret == -1)
> 3924 {
> 3925 printf ("accessing fd %d for pid %d failed: %s (%d)\n", ================> here always returns -EIO (5) errno.
> 3926 fd, inferior_ptid.pid (),
> 3927 safe_strerror (errno), errno);
> 3928 return TARGET_XFER_EOF;
> 3929 }
>
> any configure I missed in new GDB12? or new ptrace way needed?
In prior GDB versions, GDB would always use PTRACE_PEEKTEXT/PTRACE_POKETEXT for memory accesses (< 3 * sizeof(long)).
If the access was larger, then it would first try /proc/pid/mem, and if that failed, would would try with
PTRACE_PEEKTEXT/PTRACE_POKETEXT. GDB 12 always goes straight to /proc/pid/mem, and the PTRACE_PEEKTEXT/PTRACE_POKETEXT
fallback was removed. This was done because /proc/pid/mem lets you access memory even if the ptracee is not stopped,
while ptrace fails in that case.
I'd debug gdb10, and see how does linux_nat_target::xfer_partial manage to read memory there, see if the /proc access
always fails there.
If that is the case, then the next question would be, why does it fail in the first place?
^ permalink raw reply [flat|nested] 3+ messages in thread
* 回复:why ptrace read failed to read debugging process memory?
2022-03-10 10:05 ` Pedro Alves
@ 2022-03-10 10:34 ` 周春明(日月)
0 siblings, 0 replies; 3+ messages in thread
From: 周春明(日月) @ 2022-03-10 10:34 UTC (permalink / raw)
To: Pedro Alves, Simon Marchi, Gdb-patches, gdb-patches
Cc: Louis-He, Dominique Quatravaux, Sam Warner
------------------------------------------------------------------
发件人:Pedro Alves <pedro@palves.net>
发送时间:2022年3月10日(星期四) 18:05
收件人:周春明(日月) <riyue.zcm@alibaba-inc.com>; Simon Marchi <simon.marchi@polymtl.ca>; Gdb-patches <gdb-patches-bounces+riyue.zcm=alibaba-inc.com@sourceware.org>; gdb-patches <gdb-patches@sourceware.org>
抄 送:Louis-He <1726110778@qq.com>; Dominique Quatravaux <dominique.quatravaux@epfl.ch>; Sam Warner <samuel.r.warner@me.com>
主 题:Re: why ptrace read failed to read debugging process memory?
On 2022-03-10 06:40, 周春明(日月) via Gdb-patches wrote:
> Hi GDB maintainers,
> I tried update our gdb10 to gdb12, but I found new gdb seems cannot pread debugging process memory.
>
> 3897 linux_proc_xfer_memory_partial (gdb_byte *readbuf, const gdb_byte *writebuf,
> 3898 ULONGEST offset, LONGEST len,
> 3899 ULONGEST *xfered_len)
> 3900 {
> 3901 ssize_t ret;
> 3902 auto iter = proc_mem_file_map.find (inferior_ptid.pid ());
> 3903 if (iter == proc_mem_file_map.end ())
> 3904 return TARGET_XFER_EOF;
> 3905
> 3906 int fd = iter->second.fd ();
> 3907
> 3908 gdb_assert (fd != -1);
> 3909
> 3910 /* Use pread64/pwrite64 if available, since they save a syscall and can
> 3911 handle 64-bit offsets even on 32-bit platforms (for instance, SPARC
> 3912 debugging a SPARC64 application). */
> 3913 #ifdef HAVE_PREAD64
> 3914 ret = (readbuf ? pread64 (fd, readbuf, len, offset)
> 3915 : pwrite64 (fd, writebuf, len, offset));
> 3916 #else
> 3917 ret = lseek (fd, offset, SEEK_SET);
> 3918 if (ret != -1)
> 3919 ret = (readbuf ? read (fd, readbuf, len)
> 3920 : write (fd, writebuf, len));
> 3921 #endif
> 3922
> 3923 if (ret == -1)
> 3924 {
> 3925 printf ("accessing fd %d for pid %d failed: %s (%d)\n", ================> here always returns -EIO (5) errno.
> 3926 fd, inferior_ptid.pid (),
> 3927 safe_strerror (errno), errno);
> 3928 return TARGET_XFER_EOF;
> 3929 }
>
> any configure I missed in new GDB12? or new ptrace way needed?
In prior GDB versions, GDB would always use PTRACE_PEEKTEXT/PTRACE_POKETEXT for memory accesses (< 3 * sizeof(long)).
If the access was larger, then it would first try /proc/pid/mem, and if that failed, would would try with
PTRACE_PEEKTEXT/PTRACE_POKETEXT. GDB 12 always goes straight to /proc/pid/mem, and the PTRACE_PEEKTEXT/PTRACE_POKETEXT
fallback was removed. This was done because /proc/pid/mem lets you access memory even if the ptracee is not stopped,
while ptrace fails in that case.
I'd debug gdb10, and see how does linux_nat_target::xfer_partial manage to read memory there, see if the /proc access
always fails there.
[David] Yeah, I did that today, the /proc access in gdb10 is successful.
I found the memaddr passed is different betwwen gdb12 and gdb10, it's 0x7fffd9000058 from bp_tgt->placed_address in gdb10, while
0x248 in gdb12. obviously, the addr is normal host address in gdb10.
So how to calculate breakpoint address is key? I guess it's need a target base in -tdep.c.
Thanks,
-David
int
default_memory_insert_breakpoint (struct gdbarch *gdbarch,
struct bp_target_info *bp_tgt)
{
CORE_ADDR addr = bp_tgt->placed_address; ==========================> this bp address is wrong in gdb12.
const unsigned char *bp;
gdb_byte *readbuf;
int bplen;
int val;
/* Determine appropriate breakpoint contents and size for this address. */
bp = gdbarch_sw_breakpoint_from_kind (gdbarch, bp_tgt->kind, &bplen);
/* Save the memory contents in the shadow_contents buffer and then
write the breakpoint instruction. */
readbuf = (gdb_byte *) alloca (bplen);
val = target_read_memory (addr, readbuf, bplen);
If that is the case, then the next question would be, why does it fail in the first place?
^ permalink raw reply [flat|nested] 3+ messages in thread
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2022-03-10 6:40 why ptrace read failed to read debugging process memory? 周春明(日月)
2022-03-10 10:05 ` Pedro Alves
2022-03-10 10:34 ` 回复:why " 周春明(日月)
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