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authorUlrich Weigand <ulrich.weigand@de.ibm.com>2014-02-04 18:44:14 +0100
committerUlrich Weigand <ulrich.weigand@de.ibm.com>2014-02-04 18:44:14 +0100
commit591a12a1d4c8843343eb999145d8bcc1efedf408 (patch)
tree3ee9bd92c9be0f9d47afdc936fc4f1995f21920a /gdb/gdbarch.h
parentcc0e89c519912e0e4e75a2fc0d836f715cdc6806 (diff)
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PowerPC64 ELFv2 ABI: skip global entry point code
This patch handles another aspect of the ELFv2 ABI, which unfortunately requires common code changes. In ELFv2, functions may provide both a global and a local entry point. The global entry point (where the function symbol points to) is intended to be used for function-pointer or cross-module (PLT) calls, and requires r12 to be set up to the entry point address itself. The local entry point (which is found at a fixed offset after the global entry point, as defined by bits in the symbol table entries' st_other field), instead expects r2 to be set up to the current TOC. Now, when setting a breakpoint on a function by name, you really want that breakpoint to trigger either way, no matter whether the function is called via its local or global entry point. Since the global entry point will always fall through into the local entry point, the way to achieve that is to simply set the breakpoint at the local entry point. One way to do that would be to have prologue parsing skip the code sequence that makes up the global entry point. Unfortunately, this does not work reliably, since -for optimized code- GDB these days will not actuall invoke the prologue parsing code but instead just set the breakpoint at the symbol address and rely on DWARF being correct at any point throughout the function ... Unfortunately, I don't really see any way to express the notion of local entry points with the current set of gdbarch callbacks. Thus this patch adds a new callback, skip_entrypoint, that is somewhat analogous to skip_prologue, but is called every time GDB needs to determine a function start address, even in those cases where GDB decides to not call skip_prologue. As a side effect, the skip_entrypoint implementation on ppc64 does not need to perform any instruction parsing; it can simply rely on the local entry point flags in the symbol table entry. With this implemented, two test cases would still fail to set the breakpoint correctly, but that's because they use the construct: gdb_test "break *hello" Now, using "*hello" explicitly instructs GDB to set the breakpoint at the numerical value of "hello" treated as function pointer, so it will by definition only hit the global entry point. I think this behaviour is unavoidable, but acceptable -- most people do not use this construct, and if they do, they get what they asked for ... In one of those two test cases, use of this construct is really not appropriate. I think this was added way back when as a means to work around prologue skipping problems on some platforms. These days that shouldn't really be necessary any more ... For the other (step-bt), we really want to make sure backtracing works on the very first instruction of the routine. To enable that test also on powerpc64le-linux, we can modify the code to call the test function via function pointer (which makes it use the global entry point in the ELFv2 ABI). gdb/ChangeLog: * gdbarch.sh (skip_entrypoint): New callback. * gdbarch.c, gdbarch.h: Regenerate. * symtab.c (skip_prologue_sal): Call gdbarch_skip_entrypoint. * infrun.c (fill_in_stop_func): Likewise. * ppc-linux-tdep.c: Include "elf/ppc64.h". (ppc_elfv2_elf_make_msymbol_special): New function. (ppc_elfv2_skip_entrypoint): Likewise. (ppc_linux_init_abi): Install them for ELFv2. gdb/testsuite/ChangeLog: * gdb.base/sigbpt.exp: Do not use "*" when setting breakpoint on a function. * gdb.base/step-bt.c: Call hello via function pointer to make sure its first instruction is executed on powerpc64le-linux.
Diffstat (limited to 'gdb/gdbarch.h')
-rw-r--r--gdb/gdbarch.h18
1 files changed, 18 insertions, 0 deletions
diff --git a/gdb/gdbarch.h b/gdb/gdbarch.h
index b28a80e..9698e11 100644
--- a/gdb/gdbarch.h
+++ b/gdb/gdbarch.h
@@ -486,6 +486,24 @@ typedef CORE_ADDR (gdbarch_skip_main_prologue_ftype) (struct gdbarch *gdbarch, C
extern CORE_ADDR gdbarch_skip_main_prologue (struct gdbarch *gdbarch, CORE_ADDR ip);
extern void set_gdbarch_skip_main_prologue (struct gdbarch *gdbarch, gdbarch_skip_main_prologue_ftype *skip_main_prologue);
+/* On some platforms, a single function may provide multiple entry points,
+ e.g. one that is used for function-pointer calls and a different one
+ that is used for direct function calls.
+ In order to ensure that breakpoints set on the function will trigger
+ no matter via which entry point the function is entered, a platform
+ may provide the skip_entrypoint callback. It is called with IP set
+ to the main entry point of a function (as determined by the symbol table),
+ and should return the address of the innermost entry point, where the
+ actual breakpoint needs to be set. Note that skip_entrypoint is used
+ by GDB common code even when debugging optimized code, where skip_prologue
+ is not used. */
+
+extern int gdbarch_skip_entrypoint_p (struct gdbarch *gdbarch);
+
+typedef CORE_ADDR (gdbarch_skip_entrypoint_ftype) (struct gdbarch *gdbarch, CORE_ADDR ip);
+extern CORE_ADDR gdbarch_skip_entrypoint (struct gdbarch *gdbarch, CORE_ADDR ip);
+extern void set_gdbarch_skip_entrypoint (struct gdbarch *gdbarch, gdbarch_skip_entrypoint_ftype *skip_entrypoint);
+
typedef int (gdbarch_inner_than_ftype) (CORE_ADDR lhs, CORE_ADDR rhs);
extern int gdbarch_inner_than (struct gdbarch *gdbarch, CORE_ADDR lhs, CORE_ADDR rhs);
extern void set_gdbarch_inner_than (struct gdbarch *gdbarch, gdbarch_inner_than_ftype *inner_than);