[PATCH v2 3/7] [gdb/tui] Improve section handling in tui_find_backward_disassembly_start_address

[Tom de Vries <tdevries@suse.de>]

Consider a hello world:
...
$ gcc hello.c -g
...
and gdb setup like this:
...
$ gdb a.out -ex "layout asm" -ex "disassemble _start"
...
which gives us the following asm window:
...
┌─────────────────────────────────────────────────────────────────────┐
│   0x401040 <_start>       endbr64                                   │
│   0x401044 <_start+4>     xor    %ebp,%ebp                          │
│   0x401046 <_start+6>     mov    %rdx,%r9                           │
│   0x401049 <_start+9>     pop    %rsi                               │
│   0x40104a <_start+10>    mov    %rsp,%rdx                          │
│   0x40104d <_start+13>    and    $0xfffffffffffffff0,%rsp           │
│   0x401051 <_start+17>    push   %rax                               │
│   0x401052 <_start+18>    push   %rsp                               │
│   0x401053 <_start+19>    xor    %r8d,%r8d                          │
│   0x401056 <_start+22>    xor    %ecx,%ecx                          │
└─────────────────────────────────────────────────────────────────────┘
...

The _start function sits at the start of the text section:
...
Disassembly of section .plt:

   ...

0000000000401030 <puts@plt>:
  401030:       ff 25 ca 2f 00 00       jmp    *0x2fca(%rip)
  401036:       68 00 00 00 00          push   $0x0
  40103b:       e9 e0 ff ff ff          jmp    401020 <_init+0x20>

Disassembly of section .text:

0000000000401040 <_start>:
...
and using <Up Arrow> we can scroll to the last instruction of the plt section,
at 0x40103b.

To make this happen:
- tui_find_backward_disassembly_start_address is called with addr == 0x401040
- lookup_minimal_symbol_by_pc_section is called with addr - 1
- minimal symbol puts@plt is found
- its address is returned and used as starting point for disassembly

However, tui_find_backward_disassembly_start_address has two types of
information it uses: minimal symbols, and sections.

If we disable the minimal symbol handling in
tui_find_backward_disassembly_start_address, we're no longer able to scroll to
the instruction at 0x40103b.

The section handling code:
....
   /* Find the section that ADDR is in, and look for the start of the
      section.  */
   struct obj_section *section = find_pc_section (addr);
   if (section != NULL)
     return section->addr ();
...
looks up the section for 0x401040, which is .text, and it returns its start
address, which is also 0x401040, signalling that we cannot scroll to before
that address.

Fix this by detecting this case and using the previous section instead:
...
  if (section != nullptr && section->addr () == addr)
    section = find_pc_section (addr - 1);
...

This happens to work because there's no section hole between sections plt and
text.
---
 gdb/tui/tui-disasm.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/gdb/tui/tui-disasm.c b/gdb/tui/tui-disasm.c
index 7dcb59e6264..3e525000efd 100644
--- a/gdb/tui/tui-disasm.c
+++ b/gdb/tui/tui-disasm.c
@@ -177,9 +177,22 @@ tui_find_backward_disassembly_start_address (CORE_ADDR addr)
   else if (msym_prev.minsym != nullptr)
     return msym_prev.value_address ();
 
-  /* Find the section that ADDR is in, and look for the start of the
-     section.  */
-  struct obj_section *section = find_pc_section (addr);
+  /* Find the first section with start address before ADDR, and use its start
+     address.  The found section may be the one containing ADDR, or the one
+     before it.  */
+  struct obj_section *section;
+  for (int offset = 0; offset <= 1; ++offset)
+    {
+      section = find_pc_section (addr - offset);
+      if (offset == 0 && section != nullptr && section->addr () == addr)
+	{
+	  /* If ADDR is the start of its section, use ADDR - 1.  */
+	  continue;
+	}
+
+      break;
+    }
+
   if (section != NULL)
     return section->addr ();
 
-- 
2.51.0