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https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=31a56a22c45d76df4c597439f337e3f75ac3065c
https://sourceware.org/bugzilla/show_bug.cgi?id=30525
https://bugs.gentoo.org/907906
From 31a56a22c45d76df4c597439f337e3f75ac3065c Mon Sep 17 00:00:00 2001
From: Pedro Alves <pedro@palves.net>
Date: Wed, 7 Jun 2023 10:38:14 +0100
Subject: [PATCH] Linux: Avoid pread64/pwrite64 for high memory addresses (PR
gdb/30525)
Since commit 05c06f318fd9 ("Linux: Access memory even if threads are
running"), GDB prefers pread64/pwrite64 to access inferior memory
instead of ptrace. That change broke reading shared libraries on
SPARC64 Linux, as reported by PR gdb/30525 ("gdb cannot read shared
libraries on SPARC64").
On SPARC64 Linux, surprisingly (to me), userspace shared libraries are
mapped at high 64-bit addresses:
(gdb) info sharedlibrary
Cannot access memory at address 0xfff80001002011e0
Cannot access memory at address 0xfff80001002011d8
Cannot access memory at address 0xfff80001002011d8
From To Syms Read Shared Object Library
0xfff80001000010a0 0xfff8000100021f80 Yes (*) /lib64/ld-linux.so.2
(*): Shared library is missing debugging information.
Those addresses are 64-bit addresses with the high bits set. When
interpreted as signed, they're negative.
The Linux kernel rejects pread64/pwrite64 if the offset argument of
type off_t (a signed type) is negative, which happens if the memory
address we're accessing has its high bit set. See
linux/fs/read_write.c sys_pread64 and sys_pwrite64 in Linux.
Thankfully, lseek does not fail in that situation. So the fix is to
use the 'lseek + read|write' path if the offset would be negative.
Fix this in both native GDB and GDBserver.
Tested on a SPARC64 GNU/Linux and x86-64 GNU/Linux.
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30525
Change-Id: I79c724f918037ea67b7396fadb521bc9d1b10dc5
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -3909,18 +3909,26 @@ linux_proc_xfer_memory_partial_fd (int fd, int pid,
gdb_assert (fd != -1);
- /* Use pread64/pwrite64 if available, since they save a syscall and can
- handle 64-bit offsets even on 32-bit platforms (for instance, SPARC
- debugging a SPARC64 application). */
+ /* Use pread64/pwrite64 if available, since they save a syscall and
+ can handle 64-bit offsets even on 32-bit platforms (for instance,
+ SPARC debugging a SPARC64 application). But only use them if the
+ offset isn't so high that when cast to off_t it'd be negative, as
+ seen on SPARC64. pread64/pwrite64 outright reject such offsets.
+ lseek does not. */
#ifdef HAVE_PREAD64
- ret = (readbuf ? pread64 (fd, readbuf, len, offset)
- : pwrite64 (fd, writebuf, len, offset));
-#else
- ret = lseek (fd, offset, SEEK_SET);
- if (ret != -1)
- ret = (readbuf ? read (fd, readbuf, len)
- : write (fd, writebuf, len));
+ if ((off_t) offset >= 0)
+ ret = (readbuf != nullptr
+ ? pread64 (fd, readbuf, len, offset)
+ : pwrite64 (fd, writebuf, len, offset));
+ else
#endif
+ {
+ ret = lseek (fd, offset, SEEK_SET);
+ if (ret != -1)
+ ret = (readbuf != nullptr
+ ? read (fd, readbuf, len)
+ : write (fd, writebuf, len));
+ }
if (ret == -1)
{
--- a/gdbserver/linux-low.cc
+++ b/gdbserver/linux-low.cc
@@ -5377,21 +5377,26 @@ proc_xfer_memory (CORE_ADDR memaddr, unsigned char *readbuf,
{
int bytes;
- /* If pread64 is available, use it. It's faster if the kernel
- supports it (only one syscall), and it's 64-bit safe even on
- 32-bit platforms (for instance, SPARC debugging a SPARC64
- application). */
+ /* Use pread64/pwrite64 if available, since they save a syscall
+ and can handle 64-bit offsets even on 32-bit platforms (for
+ instance, SPARC debugging a SPARC64 application). But only
+ use them if the offset isn't so high that when cast to off_t
+ it'd be negative, as seen on SPARC64. pread64/pwrite64
+ outright reject such offsets. lseek does not. */
#ifdef HAVE_PREAD64
- bytes = (readbuf != nullptr
- ? pread64 (fd, readbuf, len, memaddr)
- : pwrite64 (fd, writebuf, len, memaddr));
-#else
- bytes = -1;
- if (lseek (fd, memaddr, SEEK_SET) != -1)
+ if ((off_t) memaddr >= 0)
bytes = (readbuf != nullptr
- ? read (fd, readbuf, len)
- : write (fd, writebuf, len));
+ ? pread64 (fd, readbuf, len, memaddr)
+ : pwrite64 (fd, writebuf, len, memaddr));
+ else
#endif
+ {
+ bytes = -1;
+ if (lseek (fd, memaddr, SEEK_SET) != -1)
+ bytes = (readbuf != nullptr
+ ? read (fd, readbuf, len)
+ : write (fd, writebuf, len));
+ }
if (bytes < 0)
return errno;
--
2.39.3
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