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Currently, with a program built from these sources:
$ cat extern.c
void foo () {}
$ cat static.c
static void foo () {}
$ cat main.c
int main () { return 0; }
... if you set a breakpoint on "foo", like:
(gdb) break foo
.. when there's debug info, GDB creates a breakpoint with two
locations, one for each of the external and static functions.
But, when there's no debug info, GDB creates a breakpoint with a
single location, for the external foo. Vis:
$ gcc extern.c static.c main.c -o classify.nodebug
$ gcc extern.c static.c main.c -o classify.debug -g
$ gdb classify.nodebug
Reading symbols from classify.nodebug...
(No debugging symbols found in classify.nodebug)
(gdb) b foo
Breakpoint 1 at 0x40048b
(gdb) info breakpoints
Num Type Disp Enb Address What
1 breakpoint keep y 0x000000000040048b <foo+4>
(gdb)
$ gdb classify.debug
Reading symbols from classify.debug...
(gdb) b foo
Breakpoint 1 at 0x40048b: foo. (2 locations)
(gdb) info breakpoints
Num Type Disp Enb Address What
1 breakpoint keep y <MULTIPLE>
1.1 y 0x000000000040048b in foo at extern.c:1
1.2 y 0x0000000000400492 in foo at static.c:1
GDB drops the static function is search_minsyms_for_name, where at the
very end of that function we pick only the locations with highest
classification, according to classify_type.
The classify_type logic was introduced here:
https://sourceware.org/pipermail/gdb-patches/2011-December/087864.html
which said:
"Previously, linespec was trying to filter out minsyms as they were
seen. However, this isn't faithful to gdb's historical approach,
which is to priority-order minsyms; see lookup_minimal_symbol."
lookup_minimal_symbol's intro says, in the .c file:
/* Look through all the current minimal symbol tables and find the
first minimal symbol that matches NAME. If OBJF is non-NULL, limit
the search to that objfile. If SFILE is non-NULL, the only file-scope
symbols considered will be from that source file (global symbols are
still preferred). Returns a pointer to the minimal symbol that
matches, or NULL if no match is found.
Note: One instance where there may be duplicate minimal symbols with
the same name is when the symbol tables for a shared library and the
symbol tables for an executable contain global symbols with the same
names (the dynamic linker deals with the duplication).
It's also possible to have minimal symbols with different mangled
names, but identical demangled names. For example, the GNU C++ v3
ABI requires the generation of two (or perhaps three) copies of
constructor functions --- "in-charge", "not-in-charge", and
"allocate" copies; destructors may be duplicated as well.
Obviously, there must be distinct mangled names for each of these,
but the demangled names are all the same: S::S or S::~S. */
struct bound_minimal_symbol
lookup_minimal_symbol (const char *name, const char *sfile,
struct objfile *objf)
{
If you look inside this function, you'll see:
/* External symbols are best. */
...
/* File-local symbols are next best. */
...
/* Symbols for shared library trampolines are next best. */
...
While this logic is good when you're looking for the single "best"
symbol by name, I question it for linespecs, since we want to set
breakpoints in all the multiple locations that match. I see no point
in hidding static functions.
Now, for breakpoints, it does make sense to filter out PLT/trampoline
symbols if we find the actual global matching function symbol.
Otherwise, if we did no filtering (i.e., just removed the
classify_type logic), you would end up with e.g.:
(gdb) b printf
Breakpoint 1 at 0x413a60 (2 locations)
(top-gdb) info breakpoints
Num Type Disp Enb Address What
1 breakpoint keep y <MULTIPLE>
1.1 y 0x0000000000413a60 <printf@plt>
1.2 y 0x00007ffff4653640 in __printf at printf.c:28
instead of this (which is what we get currently) before the shared
library is loaded (a location set in the PLT):
(gdb) b printf
Breakpoint 1 at 0x413a60
Num Type Disp Enb Address What
1 breakpoint keep y 0x0000000000413a60 <printf@plt>
and this after the library is loaded (only one location, no breakpoint
in the PLT):
(gdb) b printf
Breakpoint 1 at 0x7ffff4653640: file printf.c, line 28.
(gdb) info breakpoints
Num Type Disp Enb Address What
1 breakpoint keep y 0x00007ffff4653640 in __printf at printf.c:28
This patch fixes the missing breakpoint locations issue by replacing
the classify_type logic in linespec.c with a different logic.
Instead, discard a trampoline symbol if we also found a
global/external symbol with the same name. The patch adds a couple of
testcases testing locations in external vs static functions vs
trampolines/PLTs.
We now get:
For the msym-bp.exp testcase (extern vs static), without debug info:
(gdb) info breakpoints
Num Type Disp Enb Address What
1 breakpoint keep y <MULTIPLE>
1.1 y 0x000000000040048b <foo+4> ### missing before patch
1.2 y 0x000000000040049d <foo+4>
For the msym-bp.exp testcase (extern vs static), with debug info:
(gdb) info breakpoints
Num Type Disp Enb Address What
1 breakpoint keep y <MULTIPLE>
1.1 y 0x000000000040048b in foo at src/gdb/testsuite/gdb.base/msym-bp.c:21
1.2 y 0x000000000040049d in foo at src/gdb/testsuite/gdb.base/msym-bp-2.c:21
For the msym-bp-shl.exp testcase (static vs plt), without debug info, before running to main:
(gdb) info breakpoints
Num Type Disp Enb Address What
1 breakpoint keep y <MULTIPLE>
1.1 y 0x00000000004004e0 <foo@plt> ### missing before patch
1.2 y 0x00000000004005db <foo+4>
For the msym-bp-shl.exp testcase (static vs plt), without debug info, after running to main:
(gdb) info breakpoints
Num Type Disp Enb Address What
1 breakpoint keep y <MULTIPLE>
1.1 y 0x00000000004005db <foo+4> ### missing before patch
1.2 y 0x00007ffff7bd65de <foo+4>
For the msym-bp-shl.exp testcase (static vs plt), with debug info, before running to main:
(gdb) info breakpoints
Num Type Disp Enb Address What
1 breakpoint keep y <MULTIPLE>
1.1 y 0x00000000004004e0 <foo@plt> ### missing before patch
1.2 y 0x00000000004005db in foo at src/gdb/testsuite/gdb.base/msym-bp-shl-main-2.c:21
For the msym-bp-shl.exp testcase (static vs plt), with debug info, after running to main:
(gdb) info breakpoints
Num Type Disp Enb Address What
1 breakpoint keep y <MULTIPLE>
1.1 y 0x00000000004005db in foo at src/gdb/testsuite/gdb.base/msym-bp-shl-main-2.c:21
1.2 y 0x00007ffff7bd65de in foo at src/gdb/testsuite/gdb.base/msym-bp-shl-lib.c:21
gdb/ChangeLog:
* linespec.c (classify_mtype, compare_msyms): Delete.
(search_minsyms_for_name): Remove classification logic. Instead
filter out trampoline symbols if we also found an external
function of the same name.
gdb/testsuite/ChangeLog:
* gdb.base/msym-bp-2.c: New.
* gdb.base/msym-bp-shl-lib.c: New file.
* gdb.base/msym-bp-shl-main-2.c: New file.
* gdb.base/msym-bp-shl-main.c: New file.
* gdb.base/msym-bp-shl.exp: New file.
* gdb.base/msym-bp.c: New file.
* gdb.base/msym-bp.exp: New file.
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