[binutils-gdb] Delay checking whether /proc/pid/mem is writable (PR gdb/29907)

[binutils-gdb] Delay checking whether /proc/pid/mem is writable (PR gdb/29907)

[Pedro Alves]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=9dff6a5d54f212f875b6551444393691a2410f3e

commit 9dff6a5d54f212f875b6551444393691a2410f3e
Author: Pedro Alves <pedro@palves.net>
Date:   Fri Dec 16 12:04:54 2022 +0000

    Delay checking whether /proc/pid/mem is writable (PR gdb/29907)
    
    As of 1bcb0708f229 ("gdb/linux-nat: Check whether /proc/pid/mem is
    writable"), GDB checks if /proc/pid/mem is writable.  This is done
    early at GDB startup, in order to get a consistent warning, instead of
    a warning that depends on whenever GDB writes to inferior memory.
    
    PR gdb/29907 points out that some build systems (like QEMU's,
    apparently) may call 'gdb --version' to check GDB's presence & its
    version on the system, and that Gentoo's build process has sandboxing
    which blocks the /proc/pid/mem access and thus GDB warns, which
    results in build fails.
    
    To help with that, this patch delays the /proc/pid/mem check until we
    start or attach to an inferior.  Ends up potentially emiting a warning
    close where we already emit other ptrace- and /proc- related warnings,
    which just Feels Right.
    
    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29907
    Change-Id: I5537653ecfbbe76a04ab035e40e59d09b4980763

Diff:
---
 gdb/linux-nat.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index 1d207c4e31d..a244e03737e 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -372,6 +372,7 @@ linux_init_ptrace_procfs (pid_t pid, int attached)
   linux_enable_event_reporting (pid, options);
   linux_ptrace_init_warnings ();
   linux_proc_init_warnings ();
+  proc_mem_file_is_writable ();
 }
 
 linux_nat_target::~linux_nat_target ()
@@ -3955,7 +3956,11 @@ linux_proc_xfer_memory_partial (int pid, gdb_byte *readbuf,
    return true if so.  It wasn't writable before Linux 2.6.39, but
    there's no way to know whether the feature was backported to older
    kernels.  So we check to see if it works.  The result is cached,
-   and this is garanteed to be called once early at startup.  */
+   and this is garanteed to be called once early during inferior
+   startup, so that any warning is printed out consistently between
+   GDB invocations.  Note we don't call it during GDB startup instead
+   though, because then we might warn with e.g. just "gdb --version"
+   on sandboxed systems.  See PR gdb/29907.  */
 
 static bool
 proc_mem_file_is_writable ()
@@ -4490,8 +4495,6 @@ Enables printf debugging output."),
   sigemptyset (&blocked_mask);
 
   lwp_lwpid_htab_create ();
-
-  proc_mem_file_is_writable ();
 }