From 7e61560628d17ea6b1d8ee370f6d42694cff8758 Mon Sep 17 00:00:00 2001 From: Ulises Mendez Martinez Date: Fri, 4 Sep 2026 15:07:09 +0000 Subject: objtool: Validate disassembler headers in libopcodes probe commit 3f2de814c059 ("objtool: Fix libopcodes linking with static libraries") tested for libopcodes availability by linking a test snippet with a forward declaration of disassemble_init_for_target(). However, testing symbol linkage with an extern declaration only verifies the presence of the library (.so/.a) and bypasses checking for development headers (binutils-dev). On systems where libopcodes is present without development headers installed, the probe succeeds, enabling BUILD_DISAS. Subsequent compilation of objtool then fails: fatal error: 'bfd.h' file not found 113 | #include Additionally, the probe invokes $(HOSTCC) without $(HOSTCFLAGS), ignoring any sysroot or include flags specified for the host compiler. Fix this by including and directly in the test snippet, passing $(HOSTCFLAGS) so host compiler options are respected, and defining PACKAGE="objtool" to satisfy the configuration check in . Fixes: 3f2de814c059 ("objtool: Fix libopcodes linking with static libraries") Fixes: 436326bc525d ("objtool: fix build failure due to missing libopcodes check") Reported-by: Alice Ryhl Assisted-by: Antigravity:Gemini-Next Signed-off-by: Ulises Mendez Martinez Link: https://patch.msgid.link/20260904150710.2997558-1-umendez@google.com Signed-off-by: Josh Poimboeuf --- Documentation/admin-guide/device-mapper/zero.rst | 37 ++++++++++++++++++++++++ 1 file changed, 37 insertions(+) create mode 100644 Documentation/admin-guide/device-mapper/zero.rst (limited to 'Documentation/admin-guide/device-mapper/zero.rst') diff --git a/Documentation/admin-guide/device-mapper/zero.rst b/Documentation/admin-guide/device-mapper/zero.rst new file mode 100644 index 000000000..11fb5cf45 --- /dev/null +++ b/Documentation/admin-guide/device-mapper/zero.rst @@ -0,0 +1,37 @@ +======= +dm-zero +======= + +Device-Mapper's "zero" target provides a block-device that always returns +zero'd data on reads and silently drops writes. This is similar behavior to +/dev/zero, but as a block-device instead of a character-device. + +Dm-zero has no target-specific parameters. + +One very interesting use of dm-zero is for creating "sparse" devices in +conjunction with dm-snapshot. A sparse device reports a device-size larger +than the amount of actual storage space available for that device. A user can +write data anywhere within the sparse device and read it back like a normal +device. Reads to previously unwritten areas will return a zero'd buffer. When +enough data has been written to fill up the actual storage space, the sparse +device is deactivated. This can be very useful for testing device and +filesystem limitations. + +To create a sparse device, start by creating a dm-zero device that's the +desired size of the sparse device. For this example, we'll assume a 10TB +sparse device:: + + TEN_TERABYTES=`expr 10 \* 1024 \* 1024 \* 1024 \* 2` # 10 TB in sectors + echo "0 $TEN_TERABYTES zero" | dmsetup create zero1 + +Then create a snapshot of the zero device, using any available block-device as +the COW device. The size of the COW device will determine the amount of real +space available to the sparse device. For this example, we'll assume /dev/sdb1 +is an available 10GB partition:: + + echo "0 $TEN_TERABYTES snapshot /dev/mapper/zero1 /dev/sdb1 p 128" | \ + dmsetup create sparse1 + +This will create a 10TB sparse device called /dev/mapper/sparse1 that has +10GB of actual storage space available. If more than 10GB of data is written +to this device, it will start returning I/O errors. -- cgit v1.3.1