From c244297acbe51f1db5764966c02cdbd69927f218 Mon Sep 17 00:00:00 2001 From: Yafang Shao Date: Fri, 19 Mar 2021 18:12:46 +0800 Subject: vsprintf: dump full information of page flags in pGp Currently the pGp only shows the names of page flags, rather than the full information including section, node, zone, last cpupid and kasan tag. While it is not easy to parse these information manually because there're so many flavors. Let's interpret them in pGp as well. To be compitable with the existed format of pGp, the new introduced ones also use '|' as the separator, then the user tools parsing pGp won't need to make change, suggested by Matthew. The new information is tracked onto the end of the existed one. On example of the output in mm/slub.c as follows, - Before the patch, [ 6343.396602] Slab 0x000000004382e02b objects=33 used=3 fp=0x000000009ae06ffc flags=0x17ffffc0010200(slab|head) - After the patch, [ 8448.272530] Slab 0x0000000090797883 objects=33 used=3 fp=0x00000000790f1c26 flags=0x17ffffc0010200(slab|head|node=0|zone=2|lastcpupid=0x1fffff) The documentation and test cases are also updated. The output of the test cases as follows, [68599.816764] test_printf: loaded. [68599.819068] test_printf: all 388 tests passed [68599.830367] test_printf: unloaded. [lkp@intel.com: reported issues in the prev version in test_printf.c] Signed-off-by: Yafang Shao Cc: David Hildenbrand Cc: Joe Perches Cc: Miaohe Lin Cc: Vlastimil Babka Cc: Andy Shevchenko Cc: Matthew Wilcox Cc: Petr Mladek Cc: kernel test robot Reviewed-by: Sergey Senozhatsky Reviewed-by: Petr Mladek Signed-off-by: Petr Mladek Link: https://lore.kernel.org/r/20210319101246.73513-4-laoar.shao@gmail.com --- Documentation/core-api/printk-formats.rst | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'Documentation/core-api') diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst index 160e710d992f..00d07c7eefd4 100644 --- a/Documentation/core-api/printk-formats.rst +++ b/Documentation/core-api/printk-formats.rst @@ -540,7 +540,7 @@ Flags bitfields such as page flags, gfp_flags :: - %pGp referenced|uptodate|lru|active|private + %pGp referenced|uptodate|lru|active|private|node=0|zone=2|lastcpupid=0x1fffff %pGg GFP_USER|GFP_DMA32|GFP_NOWARN %pGv read|exec|mayread|maywrite|mayexec|denywrite -- cgit From a48849e2358ecf1a347a03b33dc27b9b2f25f8fd Mon Sep 17 00:00:00 2001 From: Vlastimil Babka Date: Thu, 25 Feb 2021 17:46:39 +0100 Subject: printk: clarify the documentation for plain pointer printing We have several modifiers for plain pointers (%p, %px and %pK) and now also the no_hash_pointers boot parameter. The documentation should help to choose which variant to use. Importantly, we should discourage %px in favor of %p (with the new boot parameter when debugging), and stress that %pK should be only used for procfs and similar files, not dmesg buffer. This patch clarifies the documentation in that regard. Signed-off-by: Vlastimil Babka Reviewed-by: Matthew Wilcox (Oracle) Reviewed-by: Petr Mladek Signed-off-by: Petr Mladek Link: https://lore.kernel.org/r/20210225164639.27212-1-vbabka@suse.cz --- Documentation/core-api/printk-formats.rst | 26 +++++++++++++++++++++++++- 1 file changed, 25 insertions(+), 1 deletion(-) (limited to 'Documentation/core-api') diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst index 160e710d992f..6724adf58082 100644 --- a/Documentation/core-api/printk-formats.rst +++ b/Documentation/core-api/printk-formats.rst @@ -79,7 +79,19 @@ Pointers printed without a specifier extension (i.e unadorned %p) are hashed to prevent leaking information about the kernel memory layout. This has the added benefit of providing a unique identifier. On 64-bit machines the first 32 bits are zeroed. The kernel will print ``(ptrval)`` until it -gathers enough entropy. If you *really* want the address see %px below. +gathers enough entropy. + +When possible, use specialised modifiers such as %pS or %pB (described below) +to avoid the need of providing an unhashed address that has to be interpreted +post-hoc. If not possible, and the aim of printing the address is to provide +more information for debugging, use %p and boot the kernel with the +``no_hash_pointers`` parameter during debugging, which will print all %p +addresses unmodified. If you *really* always want the unmodified address, see +%px below. + +If (and only if) you are printing addresses as a content of a virtual file in +e.g. procfs or sysfs (using e.g. seq_printf(), not printk()) read by a +userspace process, use the %pK modifier described below instead of %p or %px. Error Pointers -------------- @@ -139,6 +151,11 @@ For printing kernel pointers which should be hidden from unprivileged users. The behaviour of %pK depends on the kptr_restrict sysctl - see Documentation/admin-guide/sysctl/kernel.rst for more details. +This modifier is *only* intended when producing content of a file read by +userspace from e.g. procfs or sysfs, not for dmesg. Please refer to the +section about %p above for discussion about how to manage hashing pointers +in printk(). + Unmodified Addresses -------------------- @@ -153,6 +170,13 @@ equivalent to %lx (or %lu). %px is preferred because it is more uniquely grep'able. If in the future we need to modify the way the kernel handles printing pointers we will be better equipped to find the call sites. +Before using %px, consider if using %p is sufficient together with enabling the +``no_hash_pointers`` kernel parameter during debugging sessions (see the %p +description above). One valid scenario for %px might be printing information +immediately before a panic, which prevents any sensitive information to be +exploited anyway, and with %px there would be no need to reproduce the panic +with no_hash_pointers. + Pointer Differences ------------------- -- cgit