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inf-ptrace: Do not stop memory transfers after a single word 

When inf_ptrace_xfer_partial performs a memory transfer via ptrace with
PT_READ_I, PT_WRITE_I (aka PTRACE_PEEKTEXT, PTRACE_POKETEXT), etc., then
it currently transfers at most one word.  This behavior yields degraded
performance, particularly if the caller has significant preparation work
for each invocation.  And indeed it has for writing, in
memory_xfer_partial in target.c, where all of the remaining data to be
transferred is copied to a temporary buffer each time, for breakpoint
shadow handling.  Thus large writes have quadratic runtime and can take
hours.

Note: On GNU/Linux targets GDB usually does not use
inf_ptrace_xfer_partial for large memory transfers, but attempts a single
read/write from/to /proc/<pid>/mem instead.  However, the kernel may
reject writes to /proc/<pid>/mem (such as kernels prior to 2.6.39), or
/proc may not be mounted.  In both cases GDB falls back to the ptrace
mechanism.

This patch fixes the performance issue by attempting to fulfill the whole
transfer request in inf_ptrace_xfer_partial, using a loop around the
ptrace call.

gdb/ChangeLog:

	PR gdb/21220
	* inf-ptrace.c (inf_ptrace_xfer_partial): In "case
	TARGET_OBJECT_MEMORY", extract the logic for ptrace peek/poke...
	(inf_ptrace_peek_poke): ...here.  New function.  Now also loop
	over ptrace peek/poke until end of buffer or error.
This commit is contained in:
Andreas Arnez 2017-03-14 19:20:46 +01:00
parent d47c3ff7d5
commit 87c336f60e
2 changed files with 77 additions and 73 deletions
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8
gdb/ChangeLog
View File

@ -1,3 +1,11 @@
2017-03-14 Andreas Arnez <arnez@linux.vnet.ibm.com>
PR gdb/21220
* inf-ptrace.c (inf_ptrace_xfer_partial): In "case
TARGET_OBJECT_MEMORY", extract the logic for ptrace peek/poke...
(inf_ptrace_peek_poke): ...here. New function. Now also loop
over ptrace peek/poke until end of buffer or error.
2017-03-14 Simon Marchi <simon.marchi@ericsson.com>
* parse.c (length_of_subexp): Make static.

142
gdb/inf-ptrace.c
View File

@ -446,6 +446,72 @@ inf_ptrace_wait (struct target_ops *ops,
return pid_to_ptid (pid);
}
/* Transfer data via ptrace into process PID's memory from WRITEBUF, or
from process PID's memory into READBUF. Start at target address ADDR
and transfer up to LEN bytes. Exactly one of READBUF and WRITEBUF must
be non-null. Return the number of transferred bytes. */
static ULONGEST
inf_ptrace_peek_poke (pid_t pid, gdb_byte *readbuf,
const gdb_byte *writebuf,
ULONGEST addr, ULONGEST len)
{
ULONGEST n;
unsigned int chunk;
/* We transfer aligned words. Thus align ADDR down to a word
boundary and determine how many bytes to skip at the
beginning. */
unsigned int skip = addr & (sizeof (PTRACE_TYPE_RET) - 1);
addr -= skip;
for (n = 0;
n < len;
n += chunk, addr += sizeof (PTRACE_TYPE_RET), skip = 0)
{
/* Restrict to a chunk that fits in the current word. */
chunk = std::min (sizeof (PTRACE_TYPE_RET) - skip, len - n);
/* Use a union for type punning. */
union
{
PTRACE_TYPE_RET word;
gdb_byte byte[sizeof (PTRACE_TYPE_RET)];
} buf;
/* Read the word, also when doing a partial word write. */
if (readbuf != NULL || chunk < sizeof (PTRACE_TYPE_RET))
{
errno = 0;
buf.word = ptrace (PT_READ_I, pid,
(PTRACE_TYPE_ARG3)(uintptr_t) addr, 0);
if (errno != 0)
break;
if (readbuf != NULL)
memcpy (readbuf + n, buf.byte + skip, chunk);
}
if (writebuf != NULL)
{
memcpy (buf.byte + skip, writebuf + n, chunk);
errno = 0;
ptrace (PT_WRITE_D, pid, (PTRACE_TYPE_ARG3)(uintptr_t) addr,
buf.word);
if (errno != 0)
{
/* Using the appropriate one (I or D) is necessary for
Gould NP1, at least. */
errno = 0;
ptrace (PT_WRITE_I, pid, (PTRACE_TYPE_ARG3)(uintptr_t) addr,
buf.word);
if (errno != 0)
break;
}
}
}
return n;
}
/* Implement the to_xfer_partial target_ops method. */
static enum target_xfer_status
@ -491,79 +557,9 @@ inf_ptrace_xfer_partial (struct target_ops *ops, enum target_object object,
return TARGET_XFER_EOF;
}
#endif
{
union
{
PTRACE_TYPE_RET word;
gdb_byte byte[sizeof (PTRACE_TYPE_RET)];
} buffer;
ULONGEST rounded_offset;
ULONGEST partial_len;
/* Round the start offset down to the next long word
boundary. */
rounded_offset = offset & -(ULONGEST) sizeof (PTRACE_TYPE_RET);
/* Since ptrace will transfer a single word starting at that
rounded_offset the partial_len needs to be adjusted down to
that (remember this function only does a single transfer).
Should the required length be even less, adjust it down
again. */
partial_len = (rounded_offset + sizeof (PTRACE_TYPE_RET)) - offset;
if (partial_len > len)
partial_len = len;
if (writebuf)
{
/* If OFFSET:PARTIAL_LEN is smaller than
ROUNDED_OFFSET:WORDSIZE then a read/modify write will
be needed. Read in the entire word. */
if (rounded_offset < offset
|| (offset + partial_len
< rounded_offset + sizeof (PTRACE_TYPE_RET)))
/* Need part of initial word -- fetch it. */
buffer.word = ptrace (PT_READ_I, pid,
(PTRACE_TYPE_ARG3)(uintptr_t)
rounded_offset, 0);
/* Copy data to be written over corresponding part of
buffer. */
memcpy (buffer.byte + (offset - rounded_offset),
writebuf, partial_len);
errno = 0;
ptrace (PT_WRITE_D, pid,
(PTRACE_TYPE_ARG3)(uintptr_t)rounded_offset,
buffer.word);
if (errno)
{
/* Using the appropriate one (I or D) is necessary for
Gould NP1, at least. */
errno = 0;
ptrace (PT_WRITE_I, pid,
(PTRACE_TYPE_ARG3)(uintptr_t)rounded_offset,
buffer.word);
if (errno)
return TARGET_XFER_EOF;
}
}
if (readbuf)
{
errno = 0;
buffer.word = ptrace (PT_READ_I, pid,
(PTRACE_TYPE_ARG3)(uintptr_t)rounded_offset,
0);
if (errno)
return TARGET_XFER_EOF;
/* Copy appropriate bytes out of the buffer. */
memcpy (readbuf, buffer.byte + (offset - rounded_offset),
partial_len);
}
*xfered_len = partial_len;
return TARGET_XFER_OK;
}
*xfered_len = inf_ptrace_peek_poke (pid, readbuf, writebuf,
offset, len);
return *xfered_len != 0 ? TARGET_XFER_OK : TARGET_XFER_EOF;
case TARGET_OBJECT_UNWIND_TABLE:
return TARGET_XFER_E_IO;