Re: [PATCH v2] fs/proc/task_mmu: fix lost maps entries above 2^63

["David Hildenbrand (Arm)" <david@kernel.org>]

On 10/13/26 17:07, Stian Halseth wrote:
> When a read of /proc/pid/{maps,smaps,numa_maps} resumes, m_start()
> rewinds *ppos to priv->last_pos so that the vma which did not fit into
> the previous buffer is shown again. It only does so for last_addr > 0,
> meaning to exclude the start of the file and SENTINEL_VMA_GATE, but
> last_addr is a signed loff_t holding a vm_end. sparc64 maps shared
> libraries, anonymous memory and the stack above 0x8000000000000000, so
> the rewind does not happen there and one entry is lost at every read()
> boundary.
> 
> Reading the maps file of a process with read sizes from 64 to 8192
> bytes in steps of 64 loses lines at 126 of the 128 sizes, smaps at 123
> and numa_maps at 97, and the proc-maps-race selftest fails 4 of its 6
> tests. Python's sys.remote_exec() reads maps with fgets() and fails
> intermittently when it misses the libpython data mapping.
> 
> Skip the rewind only for 0 and the two sentinels instead, with an
> addr_is_sentinel() helper.
> 
> Fixes: 5631da56c9a87 ("fs/proc/task_mmu: read proc/pid/maps under per-vma lock")
> Closes: https://github.com/sparclinux/issues/issues/111
> Cc: stable@vger.kernel.org
> Signed-off-by: Stian Halseth <stian@itx.no>
> ---
> 
> Notes:
>     v2: addr_is_sentinel() covering both sentinels, as the comment says
>     (David Hildenbrand). No functional change: m_start() returns early on
>     SENTINEL_VMA_END. Re-tested on the T4 (7.2.10): maps and smaps read at
>     128 sizes, 200 and 1000 mappings, no lost lines.
>     v1: https://lore.kernel.org/all/20261011162019.922436-1-stian@itx.no/
>     
>     Tested on a SPARC T7-1 (sun4v, 7.3-rc5), a T4-1 (7.2.9) and a Sun Fire
>     V240 (sun4u, 7.3-rc5): no lost lines at any read size afterwards, and
>     Python's sys.remote_exec() test passes 20/20. On an amd64 KVM guest maps,
>     smaps, numa_maps and the proc selftests are unchanged.
> 
>  fs/proc/task_mmu.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 5c54aebe21182..9fd6a360919b9 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -32,6 +32,11 @@
>  #define SENTINEL_VMA_END	-1
>  #define SENTINEL_VMA_GATE	-2
>  
> +static inline bool addr_is_sentinel(loff_t addr)

Nit: no need for the "inline". The compiler is smart enough :)

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David