All of lore.kernel.org
 help / color / mirror / Atom feed
From: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
To: "Christian König" <christian.koenig@amd.com>,
	Intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org,
	Tvrtko Ursulin <tvrtko.ursulin@intel.com>,
	Daniel Vetter <daniel.vetter@ffwll.ch>,
	Sumit Semwal <sumit.semwal@linaro.org>,
	linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org
Subject: Re: [PATCH] dma-resv: Fix dma_resv_get_fences and dma_resv_copy_fences after conversion
Date: Fri, 8 Oct 2021 11:49:43 +0100	[thread overview]
Message-ID: <67f413c4-b654-c7ea-bc4f-6b42418c7486@linux.intel.com> (raw)
In-Reply-To: <9eca89ab-f954-8b2a-7af5-b4a63b90eed0@amd.com>


On 08/10/2021 11:21, Christian König wrote:
> Am 08.10.21 um 11:50 schrieb Tvrtko Ursulin:
>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>
>> Cache the count of shared fences in the iterator to avoid dereferencing
>> the dma_resv_object outside the RCU protection. Otherwise iterator and 
>> its
>> users can observe an incosistent state which makes it impossible to use
>> safely.
> 
> Ah, of course! I've been staring at the code the whole morning and 
> couldn't see it.
> 
> Going to write a testcase to cover that.
> 
>> Such as:
>>
>> <6> [187.517041] [IGT] gem_sync: executing
>> <7> [187.536343] i915 0000:00:02.0: [drm:i915_gem_context_create_ioctl 
>> [i915]] HW context 1 created
>> <7> [187.536793] i915 0000:00:02.0: [drm:i915_gem_context_create_ioctl 
>> [i915]] HW context 1 created
>> <6> [187.551235] [IGT] gem_sync: starting subtest basic-many-each
>> <1> [188.935462] BUG: kernel NULL pointer dereference, address: 
>> 0000000000000010
>> <1> [188.935485] #PF: supervisor write access in kernel mode
>> <1> [188.935495] #PF: error_code(0x0002) - not-present page
>> <6> [188.935504] PGD 0 P4D 0
>> <4> [188.935512] Oops: 0002 [#1] PREEMPT SMP NOPTI
>> <4> [188.935521] CPU: 2 PID: 1467 Comm: gem_sync Not tainted 
>> 5.15.0-rc4-CI-Patchwork_21264+ #1
>> <4> [188.935535] Hardware name:  /NUC6CAYB, BIOS 
>> AYAPLCEL.86A.0049.2018.0508.1356 05/08/2018
>> <4> [188.935546] RIP: 0010:dma_resv_get_fences+0x116/0x2d0
>> <4> [188.935560] Code: 10 85 c0 7f c9 be 03 00 00 00 e8 15 8b df ff eb 
>> bd e8 8e c6 ff ff eb b6 41 8b 04 24 49 8b 55 00 48 89 e7 8d 48 01 41 
>> 89 0c 24 <4c> 89 34 c2 e8 41 f2 ff ff 49 89 c6 48 85 c0 75 8c 48 8b 44 
>> 24 10
>> <4> [188.935583] RSP: 0018:ffffc900011dbcc8 EFLAGS: 00010202
>> <4> [188.935593] RAX: 0000000000000000 RBX: 00000000ffffffff RCX: 
>> 0000000000000001
>> <4> [188.935603] RDX: 0000000000000010 RSI: ffffffff822e343c RDI: 
>> ffffc900011dbcc8
>> <4> [188.935613] RBP: ffffc900011dbd48 R08: ffff88812d255bb8 R09: 
>> 00000000fffffffe
>> <4> [188.935623] R10: 0000000000000001 R11: 0000000000000000 R12: 
>> ffffc900011dbd44
>> <4> [188.935633] R13: ffffc900011dbd50 R14: ffff888113d29cc0 R15: 
>> 0000000000000000
>> <4> [188.935643] FS:  00007f68d17e9700(0000) GS:ffff888277900000(0000) 
>> knlGS:0000000000000000
>> <4> [188.935655] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> <4> [188.935665] CR2: 0000000000000010 CR3: 000000012d0a4000 CR4: 
>> 00000000003506e0
>> <4> [188.935676] Call Trace:
>> <4> [188.935685]  i915_gem_object_wait+0x1ff/0x410 [i915]
>> <4> [188.935988]  i915_gem_wait_ioctl+0xf2/0x2a0 [i915]
>> <4> [188.936272]  ? i915_gem_object_wait+0x410/0x410 [i915]
>> <4> [188.936533]  drm_ioctl_kernel+0xae/0x140
>> <4> [188.936546]  drm_ioctl+0x201/0x3d0
>> <4> [188.936555]  ? i915_gem_object_wait+0x410/0x410 [i915]
>> <4> [188.936820]  ? __fget_files+0xc2/0x1c0
>> <4> [188.936830]  ? __fget_files+0xda/0x1c0
>> <4> [188.936839]  __x64_sys_ioctl+0x6d/0xa0
>> <4> [188.936848]  do_syscall_64+0x3a/0xb0
>> <4> [188.936859]  entry_SYSCALL_64_after_hwframe+0x44/0xae
>>
>> If the shared object has changed during the RCU unlocked period
>> callers will correctly handle the restart on the next iteration.
>>
>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>> Fixes: 96601e8a4755 ("dma-buf: use new iterator in dma_resv_copy_fences")
>> Fixes: d3c80698c9f5 ("dma-buf: use new iterator in dma_resv_get_fences 
>> v3")
>> Closes: 
>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.freedesktop.org%2Fdrm%2Fintel%2F-%2Fissues%2F4274&amp;data=04%7C01%7Cchristian.koenig%40amd.com%7C0a73b5d07f5f44cdc5a808d98a4109f9%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637692834972816537%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C2000&amp;sdata=jhcO2Q8bGhLTW7b4%2BNn4TE3UCwBbAcQVuceJEwDK0fg%3D&amp;reserved=0 
>>
>> Cc: Christian König <christian.koenig@amd.com>
>> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
>> Cc: Sumit Semwal <sumit.semwal@linaro.org>
>> Cc: linux-media@vger.kernel.org
>> Cc: dri-devel@lists.freedesktop.org
>> Cc: linaro-mm-sig@lists.linaro.org
> 
> Maybe we should remove cursor->fences altogether, but either way the 
> patch is Reviewed-by: Christian König <christian.koenig@amd.com>
> 
> Please push to drm-misc-next ASAP.

Not sure I can or if my push permissions are limited to Intel branches. 
I can try once CI gives a green light.

Regards,

Tvrtko

> 
> Thanks,
> Christian.
> 
>> ---
>>   drivers/dma-buf/dma-resv.c | 18 ++++++++++--------
>>   include/linux/dma-resv.h   |  5 ++++-
>>   2 files changed, 14 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
>> index a480af9581bd..7b6d881c8904 100644
>> --- a/drivers/dma-buf/dma-resv.c
>> +++ b/drivers/dma-buf/dma-resv.c
>> @@ -333,10 +333,14 @@ static void 
>> dma_resv_iter_restart_unlocked(struct dma_resv_iter *cursor)
>>   {
>>       cursor->seq = read_seqcount_begin(&cursor->obj->seq);
>>       cursor->index = -1;
>> -    if (cursor->all_fences)
>> +    cursor->shared_count = 0;
>> +    if (cursor->all_fences) {
>>           cursor->fences = dma_resv_shared_list(cursor->obj);
>> -    else
>> +        if (cursor->fences)
>> +            cursor->shared_count = cursor->fences->shared_count;
>> +    } else {
>>           cursor->fences = NULL;
>> +    }
>>       cursor->is_restarted = true;
>>   }
>> @@ -363,7 +367,7 @@ static void dma_resv_iter_walk_unlocked(struct 
>> dma_resv_iter *cursor)
>>                   continue;
>>           } else if (!cursor->fences ||
>> -               cursor->index >= cursor->fences->shared_count) {
>> +               cursor->index >= cursor->shared_count) {
>>               cursor->fence = NULL;
>>               break;
>> @@ -448,10 +452,8 @@ int dma_resv_copy_fences(struct dma_resv *dst, 
>> struct dma_resv *src)
>>               dma_resv_list_free(list);
>>               dma_fence_put(excl);
>> -            if (cursor.fences) {
>> -                unsigned int cnt = cursor.fences->shared_count;
>> -
>> -                list = dma_resv_list_alloc(cnt);
>> +            if (cursor.shared_count) {
>> +                list = dma_resv_list_alloc(cursor.shared_count);
>>                   if (!list) {
>>                       dma_resv_iter_end(&cursor);
>>                       return -ENOMEM;
>> @@ -522,7 +524,7 @@ int dma_resv_get_fences(struct dma_resv *obj, 
>> struct dma_fence **fence_excl,
>>               if (fence_excl)
>>                   dma_fence_put(*fence_excl);
>> -            count = cursor.fences ? cursor.fences->shared_count : 0;
>> +            count = cursor.shared_count;
>>               count += fence_excl ? 0 : 1;
>>               /* Eventually re-allocate the array */
>> diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
>> index 8b6c20636a79..3e1bff147428 100644
>> --- a/include/linux/dma-resv.h
>> +++ b/include/linux/dma-resv.h
>> @@ -170,9 +170,12 @@ struct dma_resv_iter {
>>       /** @index: index into the shared fences */
>>       unsigned int index;
>> -    /** @fences: the shared fences */
>> +    /** @fences: the shared fences; private, *MUST* not dereference  */
>>       struct dma_resv_list *fences;
>> +    /** @shared_count: number of shared fences */
>> +    unsigned int shared_count;
>> +
>>       /** @is_restarted: true if this is the first returned fence */
>>       bool is_restarted;
>>   };
> 

WARNING: multiple messages have this Message-ID (diff)
From: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
To: "Christian König" <christian.koenig@amd.com>,
	Intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org,
	Tvrtko Ursulin <tvrtko.ursulin@intel.com>,
	Daniel Vetter <daniel.vetter@ffwll.ch>,
	Sumit Semwal <sumit.semwal@linaro.org>,
	linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org
Subject: Re: [Intel-gfx] [PATCH] dma-resv: Fix dma_resv_get_fences and dma_resv_copy_fences after conversion
Date: Fri, 8 Oct 2021 11:49:43 +0100	[thread overview]
Message-ID: <67f413c4-b654-c7ea-bc4f-6b42418c7486@linux.intel.com> (raw)
In-Reply-To: <9eca89ab-f954-8b2a-7af5-b4a63b90eed0@amd.com>


On 08/10/2021 11:21, Christian König wrote:
> Am 08.10.21 um 11:50 schrieb Tvrtko Ursulin:
>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>
>> Cache the count of shared fences in the iterator to avoid dereferencing
>> the dma_resv_object outside the RCU protection. Otherwise iterator and 
>> its
>> users can observe an incosistent state which makes it impossible to use
>> safely.
> 
> Ah, of course! I've been staring at the code the whole morning and 
> couldn't see it.
> 
> Going to write a testcase to cover that.
> 
>> Such as:
>>
>> <6> [187.517041] [IGT] gem_sync: executing
>> <7> [187.536343] i915 0000:00:02.0: [drm:i915_gem_context_create_ioctl 
>> [i915]] HW context 1 created
>> <7> [187.536793] i915 0000:00:02.0: [drm:i915_gem_context_create_ioctl 
>> [i915]] HW context 1 created
>> <6> [187.551235] [IGT] gem_sync: starting subtest basic-many-each
>> <1> [188.935462] BUG: kernel NULL pointer dereference, address: 
>> 0000000000000010
>> <1> [188.935485] #PF: supervisor write access in kernel mode
>> <1> [188.935495] #PF: error_code(0x0002) - not-present page
>> <6> [188.935504] PGD 0 P4D 0
>> <4> [188.935512] Oops: 0002 [#1] PREEMPT SMP NOPTI
>> <4> [188.935521] CPU: 2 PID: 1467 Comm: gem_sync Not tainted 
>> 5.15.0-rc4-CI-Patchwork_21264+ #1
>> <4> [188.935535] Hardware name:  /NUC6CAYB, BIOS 
>> AYAPLCEL.86A.0049.2018.0508.1356 05/08/2018
>> <4> [188.935546] RIP: 0010:dma_resv_get_fences+0x116/0x2d0
>> <4> [188.935560] Code: 10 85 c0 7f c9 be 03 00 00 00 e8 15 8b df ff eb 
>> bd e8 8e c6 ff ff eb b6 41 8b 04 24 49 8b 55 00 48 89 e7 8d 48 01 41 
>> 89 0c 24 <4c> 89 34 c2 e8 41 f2 ff ff 49 89 c6 48 85 c0 75 8c 48 8b 44 
>> 24 10
>> <4> [188.935583] RSP: 0018:ffffc900011dbcc8 EFLAGS: 00010202
>> <4> [188.935593] RAX: 0000000000000000 RBX: 00000000ffffffff RCX: 
>> 0000000000000001
>> <4> [188.935603] RDX: 0000000000000010 RSI: ffffffff822e343c RDI: 
>> ffffc900011dbcc8
>> <4> [188.935613] RBP: ffffc900011dbd48 R08: ffff88812d255bb8 R09: 
>> 00000000fffffffe
>> <4> [188.935623] R10: 0000000000000001 R11: 0000000000000000 R12: 
>> ffffc900011dbd44
>> <4> [188.935633] R13: ffffc900011dbd50 R14: ffff888113d29cc0 R15: 
>> 0000000000000000
>> <4> [188.935643] FS:  00007f68d17e9700(0000) GS:ffff888277900000(0000) 
>> knlGS:0000000000000000
>> <4> [188.935655] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> <4> [188.935665] CR2: 0000000000000010 CR3: 000000012d0a4000 CR4: 
>> 00000000003506e0
>> <4> [188.935676] Call Trace:
>> <4> [188.935685]  i915_gem_object_wait+0x1ff/0x410 [i915]
>> <4> [188.935988]  i915_gem_wait_ioctl+0xf2/0x2a0 [i915]
>> <4> [188.936272]  ? i915_gem_object_wait+0x410/0x410 [i915]
>> <4> [188.936533]  drm_ioctl_kernel+0xae/0x140
>> <4> [188.936546]  drm_ioctl+0x201/0x3d0
>> <4> [188.936555]  ? i915_gem_object_wait+0x410/0x410 [i915]
>> <4> [188.936820]  ? __fget_files+0xc2/0x1c0
>> <4> [188.936830]  ? __fget_files+0xda/0x1c0
>> <4> [188.936839]  __x64_sys_ioctl+0x6d/0xa0
>> <4> [188.936848]  do_syscall_64+0x3a/0xb0
>> <4> [188.936859]  entry_SYSCALL_64_after_hwframe+0x44/0xae
>>
>> If the shared object has changed during the RCU unlocked period
>> callers will correctly handle the restart on the next iteration.
>>
>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>> Fixes: 96601e8a4755 ("dma-buf: use new iterator in dma_resv_copy_fences")
>> Fixes: d3c80698c9f5 ("dma-buf: use new iterator in dma_resv_get_fences 
>> v3")
>> Closes: 
>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.freedesktop.org%2Fdrm%2Fintel%2F-%2Fissues%2F4274&amp;data=04%7C01%7Cchristian.koenig%40amd.com%7C0a73b5d07f5f44cdc5a808d98a4109f9%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637692834972816537%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C2000&amp;sdata=jhcO2Q8bGhLTW7b4%2BNn4TE3UCwBbAcQVuceJEwDK0fg%3D&amp;reserved=0 
>>
>> Cc: Christian König <christian.koenig@amd.com>
>> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
>> Cc: Sumit Semwal <sumit.semwal@linaro.org>
>> Cc: linux-media@vger.kernel.org
>> Cc: dri-devel@lists.freedesktop.org
>> Cc: linaro-mm-sig@lists.linaro.org
> 
> Maybe we should remove cursor->fences altogether, but either way the 
> patch is Reviewed-by: Christian König <christian.koenig@amd.com>
> 
> Please push to drm-misc-next ASAP.

Not sure I can or if my push permissions are limited to Intel branches. 
I can try once CI gives a green light.

Regards,

Tvrtko

> 
> Thanks,
> Christian.
> 
>> ---
>>   drivers/dma-buf/dma-resv.c | 18 ++++++++++--------
>>   include/linux/dma-resv.h   |  5 ++++-
>>   2 files changed, 14 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
>> index a480af9581bd..7b6d881c8904 100644
>> --- a/drivers/dma-buf/dma-resv.c
>> +++ b/drivers/dma-buf/dma-resv.c
>> @@ -333,10 +333,14 @@ static void 
>> dma_resv_iter_restart_unlocked(struct dma_resv_iter *cursor)
>>   {
>>       cursor->seq = read_seqcount_begin(&cursor->obj->seq);
>>       cursor->index = -1;
>> -    if (cursor->all_fences)
>> +    cursor->shared_count = 0;
>> +    if (cursor->all_fences) {
>>           cursor->fences = dma_resv_shared_list(cursor->obj);
>> -    else
>> +        if (cursor->fences)
>> +            cursor->shared_count = cursor->fences->shared_count;
>> +    } else {
>>           cursor->fences = NULL;
>> +    }
>>       cursor->is_restarted = true;
>>   }
>> @@ -363,7 +367,7 @@ static void dma_resv_iter_walk_unlocked(struct 
>> dma_resv_iter *cursor)
>>                   continue;
>>           } else if (!cursor->fences ||
>> -               cursor->index >= cursor->fences->shared_count) {
>> +               cursor->index >= cursor->shared_count) {
>>               cursor->fence = NULL;
>>               break;
>> @@ -448,10 +452,8 @@ int dma_resv_copy_fences(struct dma_resv *dst, 
>> struct dma_resv *src)
>>               dma_resv_list_free(list);
>>               dma_fence_put(excl);
>> -            if (cursor.fences) {
>> -                unsigned int cnt = cursor.fences->shared_count;
>> -
>> -                list = dma_resv_list_alloc(cnt);
>> +            if (cursor.shared_count) {
>> +                list = dma_resv_list_alloc(cursor.shared_count);
>>                   if (!list) {
>>                       dma_resv_iter_end(&cursor);
>>                       return -ENOMEM;
>> @@ -522,7 +524,7 @@ int dma_resv_get_fences(struct dma_resv *obj, 
>> struct dma_fence **fence_excl,
>>               if (fence_excl)
>>                   dma_fence_put(*fence_excl);
>> -            count = cursor.fences ? cursor.fences->shared_count : 0;
>> +            count = cursor.shared_count;
>>               count += fence_excl ? 0 : 1;
>>               /* Eventually re-allocate the array */
>> diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
>> index 8b6c20636a79..3e1bff147428 100644
>> --- a/include/linux/dma-resv.h
>> +++ b/include/linux/dma-resv.h
>> @@ -170,9 +170,12 @@ struct dma_resv_iter {
>>       /** @index: index into the shared fences */
>>       unsigned int index;
>> -    /** @fences: the shared fences */
>> +    /** @fences: the shared fences; private, *MUST* not dereference  */
>>       struct dma_resv_list *fences;
>> +    /** @shared_count: number of shared fences */
>> +    unsigned int shared_count;
>> +
>>       /** @is_restarted: true if this is the first returned fence */
>>       bool is_restarted;
>>   };
> 

  reply	other threads:[~2021-10-08 10:49 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-08  9:50 [PATCH] dma-resv: Fix dma_resv_get_fences and dma_resv_copy_fences after conversion Tvrtko Ursulin
2021-10-08  9:50 ` [Intel-gfx] " Tvrtko Ursulin
2021-10-08 10:21 ` Christian König
2021-10-08 10:21   ` [Intel-gfx] " Christian König
2021-10-08 10:49   ` Tvrtko Ursulin [this message]
2021-10-08 10:49     ` Tvrtko Ursulin
2021-10-08 12:19     ` Christian König
2021-10-08 12:19       ` [Intel-gfx] " Christian König
2021-10-11 12:32       ` Tvrtko Ursulin
2021-10-11 12:32         ` [Intel-gfx] " Tvrtko Ursulin
2021-10-11 14:15         ` [Linaro-mm-sig] " Christian König
2021-10-11 14:15           ` [Intel-gfx] " Christian König
2021-10-08 15:15 ` [Intel-gfx] ✗ Fi.CI.BAT: failure for " Patchwork
2021-10-08 17:33 ` [Intel-gfx] ✗ Fi.CI.CHECKPATCH: warning for dma-resv: Fix dma_resv_get_fences and dma_resv_copy_fences after conversion (rev2) Patchwork
2021-10-08 18:04 ` [Intel-gfx] ✗ Fi.CI.BAT: failure " Patchwork
2021-10-08 23:05 ` [Intel-gfx] ✗ Fi.CI.CHECKPATCH: warning for dma-resv: Fix dma_resv_get_fences and dma_resv_copy_fences after conversion (rev3) Patchwork
2021-10-08 23:36 ` [Intel-gfx] ✗ Fi.CI.BAT: failure " Patchwork
2021-10-11  9:22 ` [Intel-gfx] ✗ Fi.CI.CHECKPATCH: warning for dma-resv: Fix dma_resv_get_fences and dma_resv_copy_fences after conversion (rev4) Patchwork
2021-10-11  9:52 ` [Intel-gfx] ✓ Fi.CI.BAT: success " Patchwork
2021-10-11 12:22 ` [Intel-gfx] ✓ Fi.CI.IGT: " Patchwork

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=67f413c4-b654-c7ea-bc4f-6b42418c7486@linux.intel.com \
    --to=tvrtko.ursulin@linux.intel.com \
    --cc=Intel-gfx@lists.freedesktop.org \
    --cc=christian.koenig@amd.com \
    --cc=daniel.vetter@ffwll.ch \
    --cc=dri-devel@lists.freedesktop.org \
    --cc=linaro-mm-sig@lists.linaro.org \
    --cc=linux-media@vger.kernel.org \
    --cc=sumit.semwal@linaro.org \
    --cc=tvrtko.ursulin@intel.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.