[PATCH] debugedit: Handle unused .debug_str_offsets entries
Mark Wielaard
mark@klomp.org
Tue Apr 8 17:24:20 GMT 2025
As a sanity check we would assert when rewriting a .debug_str index we
hadn't seen while processing the DWARF. This relies on the DWARF
producer/compiler to not emit strings that aren't used. So when
processing/rewriting the .debug_str_offsets we assume all indexes
point to .debugstr offsets we have seen before.
This assumption doesn't seem to hold for clang++ at the moment. It
does put strings in the .debug_str section referenced from the
.debug_str_offsets section that aren't used anywhere else. So
debugedit will fail with an assert when processing such files.
We don't want to process the .debug_str_offsets section multiple time
and possibly have to rewrite other str references in other
sections. So instead when we encounter an unseen/unused string we
replace it with a dummy string "<debugedit>".
To help DWARF producers indentify these unused strings we keep a
reference to the original string table and produce a warning
explaining which .debug_str_offsets table at which index points to the
unseen .debug_str offset.
debugedit: Warning, .debug_str_offsets table at offset ce2d0 index
[4213] .debug_str [48cad9] entry 'CrossThreadCopierBaseHelper' unused,
replacing with '<debugedit>'
* tools/debugedit.c (debugedit_stridxentry): New static struct
stridxentry.
(struct strings): Add new field orig_data.
(create_dummy_debugedit_stridxentry): New function.
(string_find_new_entry): Add boolean argument accept_missing.
Return &debugedit_stridxentry if no existing string index
could be found.
(setup_strings): Fill in orig_data field.
(update_strings): New function to set orig_data field.
(orig_str): New function returns the original string at index.
(edit_strp): Call string_find_entry with false.
(update_str_offsets): Call string_find_entry with true, emit
warning if entry returned is &debugedit_stridxentry.
(edit_dwarf2): Call update_strings and
create_dummy_debugedit_stridxentry.
https://sourceware.org/bugzilla/show_bug.cgi?id=32845
Signed-off-by: Mark Wielaard <mark@klomp.org>
---
tools/debugedit.c | 89 +++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 83 insertions(+), 6 deletions(-)
diff --git a/tools/debugedit.c b/tools/debugedit.c
index 43f9cee65cf8..fcf8f7d6564b 100644
--- a/tools/debugedit.c
+++ b/tools/debugedit.c
@@ -144,6 +144,15 @@ struct stridxentry
Strent *entry; /* Entry in the new table. */
};
+/* Turns out we do need at least one replacement string when there are
+ strings that are never used in any of the debug sections, but that
+ turn up in the .debug_str_offsets. In that case (which really
+ shouldn't occur because it means the DWARF producer added unused
+ strings to the string table) we (warn and) replace the entry with
+ "<debugedit>". Call create_dummy_debugedit_stridxentry to add the
+ actual string. */
+static struct stridxentry debugedit_stridxentry = { 0, NULL };
+
/* Storage for new string table entries. Keep together in memory to
quickly search through them with tsearch. */
#define STRIDXENTRIES ((16 * 1024) / sizeof (struct stridxentry))
@@ -157,6 +166,7 @@ struct strentblock
struct strings
{
Strtab *str_tab; /* The new string table. */
+ Elf_Data orig_data; /* Original Elf_Data. */
char *str_buf; /* New Elf_Data d_buf. */
struct strmemblock *blocks; /* The first strmemblock. */
struct strmemblock *last_block; /* The currently used strmemblock. */
@@ -1119,6 +1129,32 @@ new_string_storage (struct strings *strings, size_t size)
return &strings->last_block->memory[stridx];
}
+static void
+create_dummy_debugedit_stridxentry (DSO *dso)
+{
+ if (debugedit_stridxentry.entry != NULL)
+ {
+ fprintf (stderr, "debugedit: "
+ "create_dummy_debugedit_stridxentry called more than once.");
+ exit (-1);
+ }
+
+ const char *dummy_name = "<debugedit>"; /* Kilroy was here */
+ const size_t dummy_size = strlen (dummy_name) + 1;
+
+ Strent *strent;
+ char* dummy_str = new_string_storage (&dso->debug_str, dummy_size);
+ if (dummy_str == NULL)
+ error (1, ENOMEM, "Couldn't allocate new string storage");
+ memcpy (dummy_str, dummy_name, dummy_size);
+ strent = strtab_add_len (dso->debug_str.str_tab, dummy_str, dummy_size);
+ if (strent == NULL)
+ error (1, ENOMEM, "Could not create new string table entry");
+
+ debugedit_stridxentry.idx = (uint32_t) -1;
+ debugedit_stridxentry.entry = strent;
+}
+
/* Comparison function used for tsearch. */
static int
strent_compare (const void *a, const void *b)
@@ -1189,12 +1225,14 @@ string_find_new_entry (struct strings *strings, size_t old_idx)
}
static struct stridxentry *
-string_find_entry (struct strings *strings, size_t old_idx)
+string_find_entry (struct strings *strings, size_t old_idx, bool accept_missing)
{
struct stridxentry **ret;
struct stridxentry key;
key.idx = old_idx;
ret = tfind (&key, &strings->strent_root, strent_compare);
+ if (accept_missing && ret == NULL)
+ return &debugedit_stridxentry;
assert (ret != NULL); /* Can only happen for a bad/non-existing old_idx. */
return *ret;
}
@@ -1291,6 +1329,8 @@ static void
setup_strings (struct strings *strings)
{
strings->str_tab = strtab_init (false);
+ /* call update_strings to fill this in. */
+ memset (&strings->orig_data, 0, sizeof (strings->orig_data));
strings->str_buf = NULL;
strings->blocks = NULL;
strings->last_block = NULL;
@@ -1299,6 +1339,21 @@ setup_strings (struct strings *strings)
strings->strent_root = NULL;
}
+static void
+update_strings (struct strings *strings, struct debug_section *sec)
+{
+ if (sec->elf_data != NULL)
+ strings->orig_data = *sec->elf_data;
+}
+
+static const char *
+orig_str (struct strings *strings, size_t idx)
+{
+ if (idx < strings->orig_data.d_size)
+ return (const char *) strings->orig_data.d_buf + idx;
+ return "<invalid>";
+}
+
/* Noop for tdestroy. */
static void free_node (void *p __attribute__((__unused__))) { }
@@ -1707,7 +1762,7 @@ edit_strp (DSO *dso, uint32_t form, unsigned char *ptr, int phase,
struct strings *strings = (form == DW_FORM_line_strp
? &dso->debug_line_str : &dso->debug_str);
idx = do_read_32_relocated (ptr, sec);
- entry = string_find_entry (strings, idx);
+ entry = string_find_entry (strings, idx, false);
new_idx = strent_offset (entry->entry);
do_write_32_relocated (ptr, new_idx);
}
@@ -2791,6 +2846,7 @@ update_str_offsets (DSO *dso)
while (ptr < endp)
{
/* Read header, unit_length, version and padding. */
+ unsigned char *index_start = ptr;
if (endp - ptr < 3 * 4)
break;
uint32_t unit_length = read_32 (ptr);
@@ -2803,13 +2859,21 @@ update_str_offsets (DSO *dso)
uint32_t padding = read_16 (ptr);
if (padding != 0)
break;
+ unsigned char *offstart = ptr;
while (ptr < endidxp)
{
struct stridxentry *entry;
size_t idx, new_idx;
idx = do_read_32_relocated (ptr, str_off_sec);
- entry = string_find_entry (&dso->debug_str, idx);
+ entry = string_find_entry (&dso->debug_str, idx, true);
+ if (entry == &debugedit_stridxentry)
+ error (0, 0, "Warning, .debug_str_offsets table at offset %zx "
+ "index [%zd] .debug_str [%zx] entry '%s' unused, "
+ "replacing with '<debugedit>'\n",
+ (index_start - str_off_sec->data),
+ (ptr - offstart) / sizeof (uint32_t), idx,
+ orig_str (&dso->debug_str, idx));
new_idx = strent_offset (entry->entry);
write_32_relocated (ptr, new_idx);
}
@@ -2963,6 +3027,9 @@ edit_dwarf2 (DSO *dso)
}
}
+ update_strings (&dso->debug_str, &debug_sections[DEBUG_STR]);
+ update_strings (&dso->debug_line_str, &debug_sections[DEBUG_LINE_STR]);
+
if (dso->ehdr.e_ident[EI_DATA] == ELFDATA2LSB)
{
do_read_16 = buf_read_ule16;
@@ -3211,7 +3278,8 @@ edit_dwarf2 (DSO *dso)
struct stridxentry *entry;
size_t idx, new_idx;
idx = do_read_32_relocated (ptr, macro_sec);
- entry = string_find_entry (&dso->debug_str, idx);
+ entry = string_find_entry (&dso->debug_str, idx,
+ false);
new_idx = strent_offset (entry->entry);
write_32_relocated (ptr, new_idx);
}
@@ -3265,8 +3333,17 @@ edit_dwarf2 (DSO *dso)
Make sure everything is in place for phase 1 updating of debug_info
references. */
if (phase == 0 && need_strp_update)
- edit_dwarf2_any_str (dso, &dso->debug_str,
- &debug_sections[DEBUG_STR]);
+ {
+ /* We might need a dummy .debug_str entry for
+ .debug_str_offsets entries of unused strings. We have to
+ add it unconditionally when there is a .debug_str_offsets
+ section because we don't know if there are any such
+ entries. */
+ if (debug_sections[DEBUG_STR_OFFSETS].data != NULL)
+ create_dummy_debugedit_stridxentry (dso);
+ edit_dwarf2_any_str (dso, &dso->debug_str,
+ &debug_sections[DEBUG_STR]);
+ }
if (phase == 0 && need_line_strp_update)
edit_dwarf2_any_str (dso, &dso->debug_line_str,
&debug_sections[DEBUG_LINE_STR]);
--
2.49.0
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