From 43968415b0025fa8e1fa5c813e53e87ae392e977 Mon Sep 17 00:00:00 2001 From: Joel Brobecker Date: Fri, 19 Sep 2014 18:00:07 -0400 Subject: [PATCH] [gdbserver/lynx] spurious failure to write in inferior memory We noticed the following error on ppc-lynx178, using just about any program: (gdb) tar remote mytarget:4444 Remote debugging using mytarget:4444 0x000100c8 in _start () (gdb) b try Breakpoint 1 at 0x10844: file try.adb, line 11. (gdb) cont Continuing. !!!-> Cannot remove breakpoints because program is no longer writable. !!!-> Further execution is probably impossible. Breakpoint 1, try () at try.adb:11 11 Local : Integer := 18; And, of course, trying to continue yielded the expected outcome: (gdb) c Continuing. warning: Error removing breakpoint 1 Cannot remove breakpoints because program is no longer writable. Further execution is probably impossible. It turns out that the problem is caused by an intentional test against a variable with an undefined value. After GDB receives notification of the inferior stopping, it tries to remove the breakpoint by sending a memory-write packet ("X10844,4:9 "). This leads us to lynx_write_memory, where it tries to split the memory-write into chunks of 4 bytes. And, in order to handle writes which are not aligned on word boundaries, we have the following code: if (skip > 0 || truncate > 0) /* We need to read the memory at this address in order to preserve the data that we are not overwriting. */ lynx_read_memory (addr, (unsigned char *) &buf, xfer_size); if (errno) return errno; (the comment explains what the code is about). Unfortunately, the not-so-glaring error that we've made here is that we're checking ERRNO regardless of whether we've called lynx_read_memory. In our case, because we are writing 4 bytes aligned on a word boundary, we do not call lynx_read_memory and therefore test an ERRNO with an undefined value. gdb/gdbserver/ChangeLog: * lynx-low.c (lynx_write_memory): Put lynx_read_memory and corresponding ERRNO check in same block. --- gdb/gdbserver/ChangeLog | 5 +++++ gdb/gdbserver/lynx-low.c | 12 +++++++----- 2 files changed, 12 insertions(+), 5 deletions(-) diff --git a/gdb/gdbserver/ChangeLog b/gdb/gdbserver/ChangeLog index 5a42642fdb1..0c25069e0cf 100644 --- a/gdb/gdbserver/ChangeLog +++ b/gdb/gdbserver/ChangeLog @@ -1,3 +1,8 @@ +2014-11-17 Joel Brobecker + + * lynx-low.c (lynx_write_memory): Put lynx_read_memory and + corresponding ERRNO check in same block. + 2014-11-12 Pedro Alves * server.c (cont_thread): Update comment. diff --git a/gdb/gdbserver/lynx-low.c b/gdb/gdbserver/lynx-low.c index 96dea03bd0d..6178e038a1b 100644 --- a/gdb/gdbserver/lynx-low.c +++ b/gdb/gdbserver/lynx-low.c @@ -688,11 +688,13 @@ lynx_write_memory (CORE_ADDR memaddr, const unsigned char *myaddr, int len) if (addr + xfer_size > memaddr + len) truncate = addr + xfer_size - memaddr - len; if (skip > 0 || truncate > 0) - /* We need to read the memory at this address in order to preserve - the data that we are not overwriting. */ - lynx_read_memory (addr, (unsigned char *) &buf, xfer_size); - if (errno) - return errno; + { + /* We need to read the memory at this address in order to preserve + the data that we are not overwriting. */ + lynx_read_memory (addr, (unsigned char *) &buf, xfer_size); + if (errno) + return errno; + } memcpy ((gdb_byte *) &buf + skip, myaddr + (addr - memaddr) + skip, xfer_size - skip - truncate); errno = 0; -- 2.30.2