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Linux: Avoid pread64/pwrite64 for high memory addresses (PR gdb/30525)

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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
This commit is contained in:
Pedro Alves
2023-06-07 10:38:14 +01:00
parent c0c3bb70f2
commit 31a56a22c4
2 changed files with 35 additions and 22 deletions
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28
gdb/linux-nat.c
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@ -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)
{